Zhou Lifang, a Chinese journalist, came to the White House press room yesterday to observe United Press International's operations, and a uniformed Secret Service policeman followed along to watch Zhou.

The policeman, who identified himself as Officer Adams, stood in the doorway outside the UPI booth keeping an eye on the visitor from Peking while Zhou watched Helen Thomas, UPI's senior White House correspondent, work. What she was working on was a story about why Adams was watching Zhou.

Initially, spokesmen for the White House and the Secret Service said the policeman was following Zhou under a long-established policy.

After about two hours, however, the policy changed. A sergeant appeared, and Adams followed him out of the press room, leaving Zhou unobserved.

Deputy press secretary Larry Speakes said it has been White House policy since 1973 to provide a police escort for visiting journalists from communist countries.

However, a Secret Service spokesman, Joe Petro, called the exercise "an honest mistake." He explained that communist journalists are escorted when they visit parts of the White House occupied by administration officials, but do not need to be watched when in the press room.

He added that the guard was no reflection on Zhou or on the New China News Agency, which he represents. "It could have been anybody," Petro said.

Zhou, who was not followed by a policeman when he visited the press room Tuesday to watch the Associated Press, said, "I was shocked that today I should be covered by police surveillance."

He said he visited the White House twice during the Carter administration, meeting the vice president once, without a police escort.

However shocked he was, Zhou seemed to find some enjoyment in the affair, and at the daily news briefing he asked Speakes several lengthy questions about his treatment.

Zhou, an official of the Chinese government agency's department of domestic news for overseas services, said he thought of leaving when he was assigned his escort, but decided it would be better to wait for the opportunity to ask Speakes questions.