BEIJING (AP) _ In poignant letters from prison, one of China's most famous dissidents sent his wife kisses and advised his sickly daughter to get more sun and exercise.

''Did you have a happy birthday?'' Xu Wenli wrote his daughter in one of five letters obtained by The Associated Press on Tuesday.

In another, he wrote gleefully: ''I had a great weekend because I got six letters.''

Xu's letters, dated since April and sent to his family in Beijing from Beijing's No. 1 Prison, are his only contact with the outside since he was barred from having visitors in 1985, four years into a 15-year sentence for counterrevolutionary activity.

Xu, 43, has spent much of the time in solitary confinement.

The former electrician was among the leaders of China's 1978-79 Democracy Wall Movement, during which Deng Xiaoping, newly in power, tolerated unprecedented open calls for multiparty democracy and other reforms.

But in early 1979, his position in the ruling Communist Party consolidated, Deng ordered a crackdown on the dozens of unofficial journals that had sprung up, and he ended the Democracy Wall itself, a brick wall in Beijing where outspoken posters went up daily.

Xu mimeographed the underground magazine April Fifth Forum in his home, in which he argued that a multiparty system was not anti-Marxist and would help the socialist cause. He was arrested in 1981 and convicted the following year.

His letters, addressed to his wife Qang Tong and 16-year-old daughter, Xu Jingjing, said nothing about his own condition. Instead, he sent a steady stream of questions and advice.

To the daughter, who has been ill for several years with a bad back, he wrote on Oct. 28: ''You should improve your daily nutrition, exercise daily and make a study plan. After you make your plans tell me or I will worry.

''Did you have a happy birthday? No matter how tired, how busy you are, try to squeeze time to sing, listen to music, play badminton, and then you will be happier and the back pain will be less. ... When you're recovered you'll be jumping around like a rabbit.''

To his wife, a teacher, he wrote, ''Your school is too far away. Try not to bicycle if possible. ... When you go home after school, is there anyone to accompany you?''

He closed, ''I'm kissing you, my dear.''

In the most recent letter, dated Dec. 2, Xu wrote, ''Regarding your letter of Nov. 12, never mind the contents. The mere fact that it's already four pages causes tears of happiness to cover my face.''

He also wrote of his pride in his wife and daughter, adding, ''Because of you, and for you, I can be at peace and accept what fate sends.''

Xu also expressed concern for his family in a ''self-defense'' smuggled from prison in 1985. Publication of the essay in Western periodicals caused authorities to cancel his visitor privileges.

Ren Wanding, a fellow participant in the Democracy Wall movement who was jailed for four years, said Xu was allowed to send a two-page letter about once every two months.

Ren, who was released in 1983, remained silent until November, when he gave foreign reporters a long essay on the Democracy Wall Movement, also called the ''Beijing Spring.''

In a new essay released Tuesday, Ren declared the 12 months beginning in November ''Beijing Spring Memorial Year'' and said his goal was to draw international attention to the plight of his jailed friends.

More than a dozen leaders of the movement remain jailed. The most famous is Wei Jingsheng, who pleaded for democracy as a ''fifth modernization'' to go along with the government's program of improving agriculture, industry, science and the military.

Although the Communist Party has encouraged greater openness on selected topics, including disclosure of official corruption, calls for any change in the party's ruling role remain forbidden.

Sporadic calls by students for democracy have been suppressed, including a series of massive nationwide demonstrations in the winter of 1986-87.

In his latest essay, Ren appealed to President-elect George Bush, who served as U.S. liaison officer to China in 1974-75.

''If you are really concerned about Chinese affairs, then I ask you to first of all be concerned about the Dzmoracy Wall personages who benefited China's social development and opening up. They have been discriminated against and watched by your friendly Chinese government,'' Ren wrote.

Aside from Ren, China's only dissident willing to be quoted as demanding multiparty democracy is internationally known astrophysicist Fang Lizhi, who believes his fame has shielded him.

Ren, a little-known accountant, said he was not so confident but that he hoped publicity would be a shield.

''I don't want China to copy Western systems or Western democracy,'' he said in an interview. ''China can work out its own democracy. China's economic liberalization inevitably must lead to greater political freedom.''