BEIJING (AP) _ As China enjoys a marked thaw in restrictions on cultural debate, an intense, voluble man with no political stature and little fame has launched a one-man crusade to open up debate on democracy and communist rule.

Sexual themes now can be portrayed on canvas and the screen, and economists can question the wisdom of state ownership of factories. But suggestions for limits to the Communist Party's power are an invitation for arrest.

Ren Wanding, a thin, shaggy-haired accountant who rarely laughs, knows it.

He spent 1979-83 in prison for having advocated multiparty democracy, and after a long silence renewed his call last month with an open letter to the U.N. Human Rights Commission and a 22-page essay on freedom.

This week, he began distributing among foreign journalists four new essays denouncing the ruling Communist Party as a narrow-minded bully and appealing to the world at large to show concern for human rights in China.

''If one just thinks about things and doesn't have the freedom to speak out, then there's no freedom,'' Ren wrote in one essay, explaining his motivation.

Aware that his own arrest now would prove the point, Ren wrote in another, ''I appeal to the power of international society for protection.''

Ren, an accountant, said in an interview that he spends most of his non- working hours writing essays and plans to distribute batches of them each month to foreign and Hong Kong media.

He said he does not expect to persuade the government to change, but wants to stimulate public debate and make China's political prisoners better known abroad, as are the dissidents in the Soviet Union.

Ren also is indignant that, as China celebrates the 10th anniversary of its opening to the outside world and economic reform under senior leader Deng Xiaoping, activists who advocated many of the same reforms remain in jail.

Those jailed, like himself, were leaders of the 1978-79 Democracy Wall Movement. During those few heady months, intellectuals debated politics in home-mimeographed journals and posters pasted on a wall in central Beijing, dubbed Democracy Wall.

Ren insisted they had an impact, despite their eventual silencing.

''If one says that Chinese society has progressed over 10 years and the economy has grown greatly, all of this is because Chinese social reformers paid the painful price of sacrificing their freedom and fortune,'' he wrote. ''Without their brave spirit, could Chinese have as much freedom of discussion and modernization as they have today? Would the ruling authorities have voluntarily granted this?''

Such Democracy Wall themes as the need for multicandidate elections, greater contact with the outside world and policies to end China's backwardness have since been adopted by the government. ''Where then is the crime?'' Ren wrote.

Despite thick glasses and a solemn manner, Ren looks younger than his 44 years. His work at as an accountant at a Beijing equipment installation company is not demanding, he said, but he has been given no chances for promotion.

Officials have refused to transfer his wife's residence permit from Tianjin to Beijing, 75 miles away. She lives illegally in Beijing with him, making it impossible for her to find work.

Ren said he has not told his 12-year-old daughter that he was in prison.

His essays are closely argued outpourings of personal frustration and outraged principles. In one, he denounced communist authorities as ''narrow- minded .... They fear the general public but bully small groups of people.''

He also denounced the practice of bypassing the courts for misdemeanor cases and allowing police to hand out sentences for ''labor education.''

''There is no way to appeal in court, and covzr-ups, false arrests and illegalities are hidden,'' he wrote. ''Why doesn't China even respect its own regulations?''

Ren, who never was tried, was ordered to undergo labor education but actually spent his four years in jail ''without labor and without education,'' he said.

Newly emerged from his long silence and without a wall or journal to proclaim his views, Ren seems uncertain how to proceed. He said he has no plans to go to campuses and address China's most volatile and receptive sector - the students - or to start up a publication.

So far, neither his employers nor police have questioned him about his new political activity.

''I'm not alone. Many people agree with me,'' he said. But he is at a loss to explain why he is the only one stepping into the dangerous limelight. ''I've always been a free-thinker,'' he said.