PHOENIX, Ariz. (AP) _ First lady Nancy Reagan, in Phoenix for her mother's funeral, was joined Wednesday by her son, Ron, her spokeswoman said.

Mrs. Reagan arrived here Tuesday, a day after her mother, Edith Luckett Davis, died at age 91.

''She has spent a lot of time making arrangements and over at her mother's apartment going through things, that sort of thing,'' spokeswoman Elaine Crispen said Wednesday night. ''Her time here will be totally private.''

The arrival of Ron Reagan on Wednesday buoyed Mrs. Reagan's spirits, Mrs. Crispen said.

''I think it's only natural that she's still a little weary from her surgery,'' Mrs. Crispen said, referring to the breast cancer operation Mrs. Reagan underwent Oct. 17. ''But we are trying to take good care of her, make sure that she gets plenty of rest. Ron, her son, arrived today and that was a good dose of medicine for her. They had dinner tonight.''

President Reagan accompanied his wife from Washington to Phoenix on Tuesday. He and Mrs. Reagan were driven to the Messinger Mortuary and Chapel in suburban Scottsdale, where the body of Mrs. Davis was taken. They then traveled to the Davis home.

The president returned to the capiital a short time later.

The funeral is scheduled for 2 p.m. Saturday at St. Thomas the Apostle Church in Phoenix. The president will return for the funeral, but his travel plans have not been announced.

Joe Brennan, a White House spokesman, said the Reagan's schedule probably would be finalized Thursday.

The Reagans' daughter, Patti, and her husband had other plans and were not expected to attend the funeral, Mrs. Crispen had said earlier.

Mrs. Davis died of a stroke, the White House said. Her husband, Dr. Loyal Davis, died in 1982.