Thomas C. Pulliam Jr., a former Prince George's County bank officer charged with receiving part of a $25,000 loan he made in the name of a fictitious person, abruptly withdrew his not guilty plea today and pleaded guilty to conspiracy.

Pulliam pleaded guilty after a surprise witness came forward and gave prosecutors evidence showing that Pulliam had admitted in sworn testimony during a 1976 child custody suit that he made two other loans in fake names while employed at a Chevy Chase bank in the mid-1970s.

Pulliam's statements about the Chevy Chase loans, contained in transcripts of his sworn testimony during the custody fight, were turned over to the government Monday night by Denis L. Murray, the attorney for Pulliam's ex-wife.

Prosecutors planned to introduce the new evidence today, but the defense realized it would probably be the deathblow to Pulliam's case. Throughout the week-long trial in U.S. District Court here, Pulliam's attorney had asked the jury to accept Pulliam's assertion that he made the $25,000 loan to a person he believed was real. Pulliam in fact made that statement under oath yesterday.

Pulliam, 37, was charged with conspiring with Gordon R. Butler, a private investigator and longtime Pulliam friend, to fraudulently obtain the $25,000 Jefferson Bank and Trust loan in May 1981 by using a fictitious name, "Robert Baker," and with submitting false financial information to get the loan.

Butler testified last week that Pulliam knew that 'Robert Baker' was an alias that Butler had used for many years in his private investigator's work.

The government contended that Pulliam used $10,000 from the $25,000 loan and $12,000 from an earlier $15,000 Jefferson loan he made in Butler's real name to help finance Pulliam's interest in a satellite television antenna business he was starting.

Jefferson, a small, Capitol Heights bank that opened in July 1980, was founded by a group of politically influential Prince George's businessmen, including County Executive Lawrence J. Hogan, County Planning Board Chairman Charles A. Dukes Jr., Liquor Board member Gerard F. Holcomb and Raymond LaPlaca, a businessman active in Republican politics.

Earlier this year, state and federal bank examiners sharply criticized Jefferson's lending policies and procedures. Federal examiners reported that nearly 40 percent of the bank's $8 million in loans were unacceptably risky and that the bank suffered from poor internal controls and incomplete loan records.

Bank officials agreed to correct any problems and said the bank was being run properly. Dukes, who until recently was the bank's chairman and is now its legal counsel, said in earlier interviews with The Washington Post that the regulators had been overzealous in their critcism of Jefferson. He said the only problems the bank had were caused by Pulliam, who was fired as executive vice president last year over the $25,000 "Robert Baker" loan.

The FBI has investigated several loans in addition to the $25,000 loan Pulliam was charged with and the government has been trying for months to persuade Pulliam to cooperate in its investigation.

Today, Assistant U.S. Attorney James P. Ulwick said in court that in return for Pulliam's plea the government had agreed not to charge him with perjury for his testimony yesterday. The government also agreed to dismiss two lesser offenses in the three-count indictment.

"That is the full and complete agreement" between the government and Pulliam, Ulwick told U.S. District Judge Joseph C. Howard, an indication that Pulliam has not agreed to help in the investigation.

Pulliam could receive five years in prison and a $10,000 fine for the conspiracy conviction. The judge did not set a sentencing date.

During 3 1/2 hours of testimony in his own defense yesterday, Pulliam said under oath repeatedly that he was fully convinced at the time he made the Jefferson loan that "Robert Baker" existed and added that he met someone claiming to be "Baker" on at least one occasion. "I had no reason to believe that 'Robert Baker' didn't exist," Pulliam testified.

Today, after entering his plea, Pulliam told Judge Howard that he knew at the time he made the Jefferson loan that Gordon Butler and "Robert Baker" were the same person.

Butler also testified that while Pulliam was a loan officer at the Bank of Bethesda in 1978 and 1979, Pulliam approved two other loans of $3,500 and $4,800 also under the fake name of "Robert Baker" to help Butler establish a financial "cover" for his detective work. Butler testified that he turned the money from the $4,800 over to Pulliam.

However, the government was unaware of the two loans Pulliam admitted making while he was a loan officer at the Chevy Chase Bank and Trust Co. in the mid-1970s. "When I called up and discovered they didn't know anything about it, I realized I had something terribly important," Murray told a reporter. At the government's request, Murray brought the deposition and hearing transcript to federal court today.

In a July 8, 1976, deposition Pulliam said he approved the two Chevy Chase loans at the insistence of his then-wife and to save his marriage. "It was fictitious names made up by my wife," Pulliam said in the deposition.

He said both loans were repaid.

The transcript of a later court hearing on Aug. 8, 1976, shows that Pulliam was again questioned about the two fictitious Chevy Chase loans and was asked: "Is there any chance of that ever happening again?"

"None," Pulliam testified.