President Reagan will deliver a speech within the next couple of months intended to "comprehensively define his environmental policies" for the first time during his administration, a White House official said yesterday.

The speech is designed as part of a counteroffensive that the administration is waging to convince the American people and Congress that the president is neither hostile nor indifferent to environmental concerns.

One White House official acknowleged yesterday that the administration was partly responsible for the public perception because Interior Secretary James G. Watt has been the most visible environmental spokesman while Reagan has never, as president, set out his environmental policies in a systematic way.

"The only person who can do this is the president," one official said.

Despite all the criticism of his administration from environmental groups, Reagan still considers himself "a conservationist," a description of himself he used Monday when he announced the nomination of William D. Ruckelshaus as director of the Environmental Protection Agency.

In a recent discussion with aides, said one of them, the president cited his environmental record as governor of California, and asked, rhetorically, "Where did I fall off the sleigh?"

No forum has been chosen for the president's environmental speech, but a first draft has been prepared, according to White House officials. In the past environmental speeches have been proposed and then discarded, but officials said the level of concern is higher now because of the allegations swirling around the EPA.

Although the speech has not been set, the administration's environmental counteroffensive is well under way. Its most important ingredient is the nomination of Ruckelshaus, to whom Reagan has given direct presidential access and a mandate to straighten out the agency.

Watt yesterday emphasized his support of the decision in a luncheon meeting with reporters, and said it was important for Ruckelshaus to have more access to the president than did his predecessor, Anne M. Burford, who resigned under fire earlier this month.

"I think there needs to be full and complete access," Watt said. "Anne probably didn't have as complete an access as she should have had."

The luncheon meeting was one of several recent initiatives by Watt aimed at correcting what he insists is a "distorted" presentation by the media of the administration's environmental record.

Yesterday Watt kept his well-known combativeness under control during most of the lunch, but he did say that environmental groups that have opposed him were interested in "partisan political activity" and weren't at all concerned about the environment.

Watt went from this lunch to a news conference, where he announced one of his pet projects, a bill intended to preserve wetland areas. He was asked there to respond to reports that some White House aides consider him a liability.

"Do I have liabilities? Yes," Watt responded. "Do I have assets? Yes. When the liabilities get bigger than the assets, we'd better do something. That hasn't come yet."

Watt said he expected that Reagan, on leaving office, would "turn around and say . . . 'Jim, turn out the lights when you come.' "

"That will be January, 1989," Watt said. "Which is to say I think he'll run and be reelected."