The chief author of the nation's landmark immigration law says illegal aliens better apply for amnesty by next Wednesday's deadline, because a Senate vote has ended any chance for an extension.

"This is it," Sen. Alan Simpson, R-Wyo., said Thursday night after helping to kill a plan that would have moved the deadline to Nov. 30. "This extraordinary act of grace will end May 4 when we are in recess."Those who sought to move the date back nearly seven months were defeated when they tried to end debate by opponents of the extension. Only 40 votes could be mustered to end the budding filibuster, and 60 were needed. Both Utah Senators Jake Garn and Orrin Hatch, Republicans, voted to continue the debate.

With the Senate adjourning Friday until May 9, supporters of the extension have no chance to try to pick up another 20 votes.

Even if the extension had passed Congress, Sen. Phil Gramm, R-Texas, said President Reagan would have vetoed the bill as suggested by Commissioner Alan C. Nelson of the Immigration and Naturalization Service.

The House passed the amnesty extension April 21 by a 213-201 vote, indicating that a veto would have been sustained there. A two-thirds vote is needed for an override.

Simpson was the chief sponsor of the 1986 law passed to control years of massive illegal immigration to the United States. The measure had two main portions: legalization for illegal aliens who came to the United States before Jan. 1, 1982 and have lived here continually since then; and financial penalties for employers who knowingly hired illegal residents.

The aliens were given a one-year period, through May 4, to apply for temporary residency and start down the longer road toward permanent residency and citizenship.

About 1.3 million applications have been received so far, and the approval rate is running about 95 percent, according to INS figures.

Those who opposed the extension argued that supporters of the change played a cruel hoax on illegal aliens by proposing legislation that had no chance of enactment.

Officials of the Senate Judiciary immigration subcommittee said Thursday that people have been calling daily asking how long the extension would last.

Even before the vote, the INS anticipated it would be swamped as the deadline approaches. The agency has already announced that any alien who shows up before the deadline can file an application now and produce the supporting documents later.

Offices will officially be open until midnight Wednesday, and the INS said anyone in line will be permitted to file an application.

Applicants have to pay a fee of $185 for individuals and $420 for a family.

Gramm said illegal aliens who fail to beat the deadline will "lose the opportunity to get the best gift America has to be in the United States legally."

"After an initial period when the law was taken seriously, the level of illegal immigration has risen by 60 percent over the last year," he said. "When people are asked why they are coming, they say they don't believe the law is going to be enforced."