The union representing D.C. police officers has endorsed City Council member Arrington Dixon for council chairman in the Sept. 12 Democratic primary, with its president saying the choice was easy because of Dixon's major opponent - council member Douglas E. Moore.

Larry L. Simons, president of International Brotherhood of Police Officers Local 442, said Dixon (D-Ward 4) has been accessible to union members and has supported measures strengthening the organization's collective bargaining power with the city.

"We can pretty much depend on him," Simons said of Dixon, whom he also termed a "top-notch" administrator. "We could go sit down and discuss things with him. We could get him to sponsor and cosponsor a bill."

"Mr. Dixon's was a very easy endorsement, owing to the fact of who else is running for chairman," Simons said. "We have been totally shut off by Douglas Moore.

"If he's chairman," Simons said of Moore (D-At Large), "he would hurt the council because he cannot work with anybody there."

Simons organization represents about 3,700 of the 4,200 uniformed city police and detectives. The decision to endorse Dixon was made last month by a unanimous vote of the organization's 15-member executive council, Simons said.

The union will probably contribute money to Dixon's campaign and perhaps hold fund raisers for Dixon, Simons said, as well as encourage its estimated 1,000 members who live in the city to vote for Dixon.

The Hatch Act prohibits city policemen from working in political campaigns. "Now," Simons quickly reminded a reporter, however, "our wives can work. I think that's one of our plans. We don't want to violate the law. But the Hatch Act does not affect our wives."

Dixon whose campaign strategy is to avoid outright denunciations of Moore, issued a press release announcing the endorsement late Thursday. "I think all of us in government realize that an efficient police force is essential to the health of the city," he said. "Over the years, I have enjoyed a very positive relationship with the D.C. Police Department."

The rationale given by Simons for union selection of Dixon echoed Dixon's campaign theme of trying to paint Moore as a political "lone ranger" who lacks the necessary support and respect of key interest groups to head effectively the city's legislature.

Moore was unavailable for comment. On Wednesday, the union announced endorsement of council member Marion Barry (D-At Large) for mayor in the Sept. 12 Democratic primary.

Simons said one of the legislative measures which gained Dixon the police officers' support was his stand in favor of giving the group increased collective bargaining power under a newly proposed civil service system for the city.

Under the present system, the union representing police officers can bargain only on the issue of wages. Under the poposed bill, a provision supported by Dixon and the union would increase that authority to include hours, occupational safety hazards, discipline and other areas, according to a Dixon committee aide.

Dixon has also proposed legislation that would clear up an impasse between the union and the city's board of labor relations concerning union representation of officers involved in minor disciplinary issues. The union claims the right to such representation while the city government disagrees. Dixon's legislation would establish that right by law.

The endorsement of Dixon comes when former Assistant Police Chief Tilmon O'Bryant is considering running for council chairman. Simons said O'Bryant, who at times angered some policemen during his tenure as assistant chief in charge of administration, would not get much support from the union.

"If Chief O'Bryant runs for chairman, it will actually be a joke in the police department," Simons said. "If I were to even attempt to get an endorsement, there would be hundreds of guys who would be furious."

The other announced candidate for chairman is the Rev. John G. Martin, pastor of Holy Comforter Baptist Church, who ran unsuccessfully for City Council at-large in the 1976 Democratic primary.