A bipartisan group of senators yesterday lambasted the acting head of the Small Business Administration for actions they said amount to attempts to dismantle the federal agency despite a recently passed law prolonging its life.

Members of the Senate Small Business Committee raked acting administrator Charles L. Heatherly over the coals for a series of actions, including the abrupt firing of six of the agency's 10 regional administrators his first day on the job, and for statements about the prospect of eliminating half of the agency's employes and about the ineptitude of Congress.

Some of the harshest criticism came from Republicans, including Sen. Rudy Boschwitz (R-Minn.), an active supporter in the past of administration proposals to abolish the SBA.

Since he took over the SBA at the beginning of April, Heatherly's actions "really have gone a long way to undermining the effectiveness of the agency," Boschwitz said. "Maybe that's what you want to achieve."

But, noting that Congress had voted to keep the SBA alive, Boschwitz said, "We're not going to see congressional intent frustrated. . . . I would hope you would go back to the agency and try to rebuild what is left of it."

Heatherly defended himself, saying that he was committed to carrying out the agency's legally mandated programs. At the same time, he said he is actively advocating President Reagan's position that the agency's loan programs should be terminated and other SBA functions moved into the Commerce Department.

He also said that some of what he called misconceptions about his aims stemmed from the news media, which took too seriously some comments he made and forced him into others, and from interest groups trying to scare the small-business constituency.

But Heatherly's explanation of his position met with acerbic comments from members of the committee.

"Either you're dissembling or you don't have the experience, you don't have a sense, perhaps, of what you're doing," Boschwitz said. "You are dismantling something."

"I am disturbed by some of the actions of the acting administrator," said Sen. Slade Gorton (R-Wash.). "Destruction of the SBA's mission and humiliation and destruction of the morale of the employes are not part of his job."

Sen. Max Baucus (D-Mont.) said he failed to understand "how you and this administration have the audacity to proceed with this dismantling when Congress writes the law."

Heatherly, a former deputy undersecretary for management in the Department of Education and editor of the Heritage Foundation's "Mandate for Leadership" report in 1980, was appointed deputy administrator of the SBA by his predecessor, James C. Sanders, shortly before Sanders left. On April 1, he was named acting administrator by the White House. Heatherly said yesterday that he was told before he accepted the job that the White House did not intend to nominate a permanent administrator.

"Is it the White House position that the agency is not deserving of an administrator?" asked Sen. Lowell Weicker (R-Conn.), chairman of the committee. Weicker charged repeatedly during the hearing that Heatherly had set out to accomplish, "de facto," changes that the White House had failed to get in law.

Sen. Bob Kasten (R-Wis.) noted that considerable effort went into the compromise that cut back SBA but kept it intact as an independent agency, and that President Reagan had signed a three-year reauthorization of the agency. "Did you miss all that?" he asked Heatherly.

"It's coming through to at least one member of this committee that there's a little bit of contempt for us" in Heatherly's attitude, Kasten said.

Heatherly said that he thought his problems stemmed from advocating a position, abolishing the SBA as an independent agency, that members of the Senate committee don't wish to see advocated this year. He said that the president's proposal had been revised from abolishing the SBA to keeping some of its functions intact but within the Commerce Department, and that the revised proposal deserved a hearing.

Weicker noted that there were no Senate sponsors for the administration's proposal but said that his committee would hold hearings if it found sponsors. He made it clear, however, that the reception would not be friendly. "You have pulled together the small-business community as nobody has ever pulled it together," he said.

Heatherly said he has tried to assuage employes' concern about their jobs while at the same time trying to educate them about Reagan's proposal.

He also said that, while he indicated in an April 2 memo to SBA employes that one of his missions is "to prepare the agency for an orderly transition to a new configuration at the Department of Commerce once the transfer of functions has been enacted into law," that he was first committed to carrying out the law that exists.