Joseph A. Cannon, head of policy and resource management at the Environmental Protection Agency, met his new boss for the first time last Friday.

In an hour-long meeting that Cannon later characterized as "mellow, very friendly," he and Administrator William D. Ruckelshaus discussed in broad terms the major issues facing the agency after more than two years of escalating controversy and bitter political infighting.

Cannon declined to give specifics on the discussion. "He was just meeting Joe Cannon," he said. "I came out of it very pleased at the opportunity to work with him."

Cannon did not know at the time that he had just become a rarity as sole survivor of a top-level purge that had claimed its latest victims hours earlier.

In similar meetings earlier Friday, Ruckelshaus had requested--reluctantly, according to several officials--resignations of the assistant administrators for air and water. Their departures leave Ruckelshaus as the only Senate-confirmed EPA official.

EPA insiders said they expect the move to be Ruckelshaus' only deck-clearing; from now on, they said, he will be too busy hoisting sails.

"He's going to move fast," a Republican Senate aide predicted earlier this month, on the day the Senate Environment and Public Works Committee gave Ruckelshaus its unanimous endorsement. "You could just see his eagerness at his confirmation hearings. He was ready to get on with the job."

Ruckelshaus is likely to sail a course he explored and charted as the EPA's first administrator.

He has begun signing on his crew, choosing from among what he told the Senate committee were "literally thousands of offers to help." EPA employes who formerly muttered darkly about the agency's poisonous atmosphere talk optimistically about recapturing the air of enthusiasm and adventure that propelled it in its halcyon days.

The turnaround began even before Ruckelshaus was sworn in last Wednesday at a White House ceremony that was, to some observers, an important bellwether.

Environmentalists, invited to the White House for the swearing-in, sat alongside industry representatives. Canadian Ambassador Allan Gotlieb was given a seat of honor, and the first charge Ruckelshaus received from President Reagan was to "tackle acid rain."

Canadian officials, who have complained bitterly about what they see as foot-dragging by the Reagan administration on the international problem of acid rain, said the symbolism was welcome.

"We have been very encouraged by signals out of the new EPA," said George Rejhon, the embassy's environmental counselor. "His first senior appointment was director of international affairs, Fitzhugh Green. We've already met with him twice, and he made it eminently clear he proposes to work closely with us."

A congressional lunch last week, attended by Green, Canadian Environmental Minister John Roberts and State Department negotiator Thomas Niles, was "remarkable for the difference in content and tone" from that under former EPA administrator Anne M. Burford, according to one participant.

Ruckelshaus also has taken steps to fulfill two pledges made in his confirmation hearings. A change in hazardous-waste cleanup policy under the $1.6 billion "Superfund" was announced the week before he took office. His first major official act was to issue a new ethics policy designed to ensure that the EPA, as he promised the Senate, will be run "in a fishbowl."

According to several EPA officials, Ruckelshaus is likely to keep reorganizations to a minimum.

"He has a strong bias against major reorganizations," one official said, noting that Ruckelshaus is aware that his time to achieve change is relatively short. "If you only have a year and a half, you don't have time to wait for people to absorb change," the official said.

Some changes are likely in the enforcement division, however, where three major reorganizations in less than a year drew sharp criticism from inside and outside the agency. Effectiveness there is essential to restoring the agency's image as a tough protector of public health and is seen as Ruckelshaus' most immediate management problem.

According to EPA officials, Courtney Price, a Transportation Department official named to replace EPA general counsel Robert M. Perry, has received Ruckelshaus' approval as special enforcement counsel.

Some of the agency's sharpest critics say they are pleased with the recent flurry of positive pronouncements, although, as one lobbyist on clean air issues said, "It takes more than a few sweet words to seduce me. But they have my attention while they whisper in my ear."