A Prince George's County police sergeant was found guilty today of preparing false tax returns in a scheme that Maryland prosecutors said he used to deduct personal expenses such as mortgage and trash removal payments by calling them church contributions.

The prosecutors said Sgt. Robert F. Fowler, 37, was the leader of a group of several police officers who claimed thousands of dollars in phony contributions to the Universal Life Church based in Modesto, Calif.

Anne Arundel County Circuit Court Judge Robert Heller found Fowler, a 13-year veteran of the police force, guilty of two misdemeanor counts of preparing false returns for other persons. Fowler faces a possible maximum sentence of six months in prison and a $1,000 fine on each count. Sentencing was tentatively scheduled for Oct. 30.

He is still on the police force pending a review of his case by the department's internal affairs unit, and he could face a disciplinary trial board.

Fowler, a "pastor" of the Universal Life Church, claimed almost $16,000 in tax-deductible contributions to the church in 1981 alone, according to prosecutors. The "church" was his home in Upper Marlboro where little religious activity occurred, prosecutors said, and the "contributions" consisted of payments on his mortgage, plus items such as lawn service, trash removal, video equipment, a power mower, a Ford van, a microwave oven and furniture for the house, as well as vacation "retreats" for Fowler and his family.

"The whole thing flies or dies on the paperwork," Fowler was quoted as telling fellow police officers in papers filed in court today.

The Universal Life Church in California had been granted tax-exempt status by the Internal Revenue Service in 1974 but was stripped of the status last month. Regardless of the California church's status, prosecutors here say, Fowler's operation was illegal because his "church" was not used exclusively for religious purposes and his contributions were for his own benefit.

The action against Fowler comes after the tax returns of scores of Maryland residents, including several Prince George's County police officers, were flagged last year by state auditors because of huge tax deductions they had claimed as "pastors" of the church. For a $35 fee, the church issues "credentials of ministry" with no religious training required. It provides instructions on how to set up local branch churches and bank accounts for processing contributions.

The Maryland comptroller's office disallowed about $180,000 in contributions to locally created branch churches, and an Anne Arundel County school teacher, David W. Vickers, 43, was charged with preparing false tax returns, the same charge leveled against Fowler. Vickers pleaded guilty in August and is scheduled to be sentenced Oct. 3.

Because a large number of Prince George's County police officers became involved as "pastors" of the Universal Life Church, former police chief John E. McHale issued a warning in 1981 that the operation might be illegal.

As part of a delicately negotiated agreement today in which Fowler pleaded not guilty but did not contest the claims against him, Assistant Attorney General Bruce C. Spizler introduced into court a statement of facts that said Fowler had designated his Upper Marlboro home as a church, naming himself as pastor, his wife, Nancy, as secretary-treasurer and fellow police officer Benny Guy Gilbreath as assistant pastor.

The three opened a church bank account and made periodic check payments from their personal accounts to the church account, marking the checks as "donations," Spizler said.

Fowler enlisted other police officers to set up similar schemes, Spizler said, and made at least three trips to West Virginia where, for a fee, he counseled a group of coal miners on how to set up local branches of the Universal Life Church.

Spizler said Fowler and other police officers, concerned that the scheme might violate the law against church contributions going to their personal benefit, devised a way of "trading" donations among different local "churches" to avoid direct receipt of donations.