The Air Transport Association of America, which represents 95 percent of the nation's commercial airlines, has decided to oppose the Reagan administration's plan to limit use of National Airport.

Association spokesman Daniel Henkin said yesterday the group will testify against the plan at public hearings in July, and will submit the plan at public hearings in July, and will submit written objections in August. "We had some serious problems with the plan," Henkin said yesterday. "We're not trying to kill it. We're just going to be presenting our views."

In a statement released after a Wednesday night meeting of representatives of 15 carriers, the association said its opposition to the new National proposal is based largely on the absence of an "economic impact analysis" and "imposition of arbitrary noise level standards." "

"The carriers agreed," the statement read, "that the proposed rule would have a substantially adverse effect on service to and from Washington."

The government proposal, announced July 8 by Transportation Secretary Drew Lewis, would limit air traffic at National by imposing an annual passenger limit that is close to the number of people using National today, continuing a ban on widebodied jets and setting maximum noise levels for individual flights. The plan also contains proposals to promote increased use of Dulles International Ariport. Both National and Dulles are owned by the federal government.

The Air Transport Association's members have rarely been able to speak with one voice on the airport issue, because the various airlines have different goals. Some airlines, particularly Eastern, are anxious to bring wide body airliners into the National terminal. Others, like American and Braniff, are eager to have nonstop flight rights presently denied them to Dallas and Houston.

But the biggest issue -- and one on which all the airlines can agree -- is that they want essentially unrestricted access to National. The new plan is difficult for them to accept in the competitive atmosphere brought on by deregulation.