China has informed the U.S. government that the jailed Chinese fiance of an American woman student has been released and will be allowed to travel to the United States, according to diplomats here.

The American student, Lisa Wichser, 31, of Indianapolis, has been separated from her fiance, Yi Xigong, 33, for more than 2 1/2 years. He was held in a reeducation labor camp following allegations that he gave secret information to Wichser.

In what appeared to U.S. officials to be a humanitarian gesture, the Chinese government released Yi in December. Last month, the government approved his request to travel to the United States. He is expected to reach the United States within a few weeks and to marry Wichser shortly thereafter.

Wichser has told officials and congressional aides who have been concerned with Yi's case that she will not discuss it until he reaches the United States. She lives in San Francisco.

In May 1982, Chinese security police arrested Wichser in Peking and held her for six days. She was accused of involvement in the theft of Chinese "state secrets."

At the time, Wichser, a doctoral candidate at the University of Denver, was teaching in Peking and gathering material for postgraduate papers on socialist economic theory. According to several of her colleagues in Peking, Wichser had applied to the Chinese authorities about two months before her arrest to marry Yi, a student whom she had met in the course of her work.

U.S. Embassy officials in Peking protested to the Chinese authorities that they had not been informed speedily of Wichser's arrest as required by a consular convention signed only shortly before the arrest. A U.S. Embassy spokesman denied that Wichser had any U.S. government connection, saying that she was a private student previously unknown to U.S. officials in Peking. Several hundred American students currently are studying in China.

Wichser was arrested at 1 a.m. on a Friday, but U.S. Embassy officials were barred from seeing her until the following Monday.

The arrest stirred considerable concern in the U.S. Congress. Former Senate majority leader Howard H. Baker Jr. is reported to have raised the issue with Chinese officials during a visit to Peking in early June.

Sen. Richard G. Lugar (R-Ind.), chairman of the Senate Foreign Relations Committee, released a statement Jan. 9 saying that for the past two years he had been working for the release of Yi so that he could come to the United States to marry Wichser. Lugar said that Wichser told him that the information that she obtained in China was openly published material.

Before President Reagan visited China last April, Lugar and Rep. Stephen J. Solarz (D-N.Y.), chairman of the House subcommittee on Asian and Pacific affairs, wrote letters to the Chinese authorities pleading for Yi's release. In the course of the Reagan visit, Secretary of State George P. Shultz delivered the two letters to a high-ranking Chinese official.

Lugar's letter said that the release of Yi "would be looked upon by people throughout the world as a gesture of good will and an opportunity to strengthen Chinese-American relations."

After being informed by the Chinese Embassy here that Yi's travel had been approved, Lugar called Wichser in San Francisco. According to the senator, "She was ecstatic, but she said she won't believe it until her fiance is here. Then they plan to go to Indianapolis and get married."

Wichser's mother, Julia, reached at her home in Noblesville, Ind., said that she was grateful to Lugar, Solarz, Shultz and the Chinese government for their efforts.

An aide to Solarz said that the congressman would not comment until Yi arrived in the United States. The aide did venture a comment of his own, however.

"It's a great love story," he said.