President Carter yesterday unveiled a comprehensive steel policy that would relax clean air deadlines in certain cases, provide tax incentives for new investment and attempt to stop unfair trade practices.

Carter called the plan, announced exactly five weeks before the presidential election, "an important event in my own life as president and, I think, in the life of our country."

The revitalization program is intended to help wipe out the industry's expected shortfall, provide relief to thousands of laid-off steelworkers and their communities and give long-range aid to the steel companies' modernization efforts to help them become more competitive in world markets.

Key elements of the plan are:

A proposed amendment to the Clean Air Act allowing the government to extend by up to three years a steel company's deadline for meeting pollution standards if the company agrees that the money saved will be used for modernization. Administration officials claimed air quality would not deteriorate.

Consideration of a similar Clean Water Act amendment was deferred pending formulation of new regulations this year.

Tax breaks, including a proposed 40 percent liberalization of depreciation rules to provide the steel industry with up to $150 million in savings by the fifth year, an extra 10 percent credit for investment in distressed areas and partial refundability -- rebates for amounts higher than tax liability -- for the regular and new investment tax credits that would give $100 million in savings to the steel companies during the first year. These tax provisions were announced earlier by the president as part of his broader business reindustrialization policy.

A modified trigger-price mechanism lasting up to five years to give U.S. and European steelmakers a chance to retool and modernize. After the five-year limit, the United States will begin vigorous enforcement of antidumping laws. Trigger prices will rise an average of 12 percent and, under special circumstances, some dumping cases can be filed without the suspension of the trigger prices.

The trigger-price mechanism, a system under which the government identifies imports likely to be dumped here, was suspended last March after U.S. Steel Corp. filed dumping complaints against seven European steelmakers. The government contended it couldn't administer trigger prices and the dumping cases at the same time.

Exploration by the White House of cooperation between the public and private sectors on research and development programs.

The plan, approved by union and industry officials and hailed by the European Economic Community, is the nation's first policy for a particular industry resulting from a tripartite committee of government, labor and industry leaders.

Carter's chief domestic adviser, Stuart E. Eizenstat, said that the tripartite concept "is an important process, and we need this cooperation to solve problems of the 1980s."

The president's proposals, Eizenstat said, should close the steel industry's annual capital shortage, predicted to be anywhere between $600 million and $3 billion.

In response to the president's plan, U.S. Steel Corp. said it will drop its complaints, ending for the moment a tense, six-month import dispute that threatened to lead to an international trade war.

The plan is a result of an election-year push for revitalizing the nation's industries. A steel tripartite committee was set up, consisting of Commerce Secretary Phillip M. Klutznick, Labor Secretary Ray Marshall, U.S. Trade Representative Reubin Askew and union and industry leaders who have agreed that U.S. steelmakers will need to invest $4 billion to $5 billion a year for the next five years to catch up with leading steel producers in Japan, Canada and Western Europe.

Relaxation of environmental deadlines in some instances was a major thrust of the president's proposal because the steel industry has complained that it can't afford to invest in pollution control equipment and modernize at the same time.

The plan is also a response to Republican presidential contender Ronald Reagan's own steel revival plan which would allow companies to write off capital assets faster than the average 12 years they now have, extend pollution control deadlines, reinstate the trigger-price mechanism and allow the industry to make a higher "reasonable profit" to generate more investment and job-producing modernization and expansion.