Defense Secretary Harold Brown yesterday warned the Senate that drastic laws to outlaw unions within the military might cause more problems than they solve.

Called before the Senate Armed Services Committee to explain how he intends to combat the potential unionizing of the military, Brown said: "I don't plan to overreact."

Pentagon rules already on the books are adequate to handle the current threat of unionization, Brown said. He promised to issue "stronger directives" to combat unionization if "a large-scale campaign" developes to organize service personnel.

"Caution should be exercised in any legislative action in this area," Brown warned the committee.

Brown and his Pentagon advisers fear that a stuff anti-union law might be declared unconstitutional by the courts and give added impetus to unions considering an organization drive within the military services.

The American Federation of Government Employees (AFL-CIO) is considering such a drive. Its executive committee this month voted to poll the union's 280,000 members on whether the AFGE should try to establish locals within the military.

Although the balloting is not expected to be finished until this summer, Sen. Strom Thurmond (R.S.C.) and 37 cosponsors already have introduced a bill in the Senate to ban any unionization of the military.

Brown and Navy Secretary W. Graham Claytor reminded the committee yesterday that President Carter and the service secretaries are opposed to unionization of the military. But Brown said the administration is not yet ready to endorse Thurmond's bill or submit one of its own.

Committee Chairman John C. Stennis (D-Miss.) said, "It's news to me that through your regulations you can take care of this problem." He warned:

"Unless we meet this" unionization threat "with firm legislation," the nation's all-volunteer force "is going to go downhill fast and prove to be a failure.

"You've got to make up your mind," Stennis told Brown, "which way you're going to go on a real bill" to prevent unionization of the military.

Otherwise, Stennis continued, "You're in for trouble, the services are in for trouble and Congress is in for trouble."

Brown said Pentagon rules already forbid collective bargaining with representatives of military personnel. Also, he said, commanders can stop organizational efforts on their bases if the activities undercut discipline.

In urging the go-slow approach toward outlawing unionization, Brown said. "The threat is prospective, not immediate. It is important for me not to overreact."

Navy Secretary Claytor suggested obtaining advice from legal experts outside the government so as not to write a bill that infringed on First Amendment rights.

Thurmond, in urging th e passage of anti-union legislation in regard to the military, said, "There is no place in the armed forces for an organization that could come between a commander and his troops."

Yesterday's committee session marked the first time Congress had conducted a formal hearing on unionization of the military.

Kenneth Blaylock, president of the union, said in a recent "Navy Times" interview that "Our reasons for organizing would not be the idealistic or philosophical reasons that some groups want us to adopt. For example, the right to challenge the legality of orders. Should we charge that hill or should we not?Should we be in this war?

Instead, said Blaylock, union locals within the military would deal in "bread-and-butter" issues such as pay and fringe benefits as well as in work assignments, disciplinary actions and change of duty stations.

An American Civil Liberties Union suit filed this month in Fayetteville, N.C., alleges that 82d Airborne Division officers unlawfully forced Army Spec. 5 Thomas L. Doran Jr. out of the service because of his unionizing activities that "were legal in all respects."