The U.S. airlift last Friday of hundreds of black Ethiopian Jews stranded in a Sudanese refugee camp came after all members of the Senate had signed a letter Feb. 21 urging President Reagan to undertake the action, Sen. Alan Cranston (D-Calif.) said yesterday.

The letter noted Sudanese President Jaafar Nimeri's declared willingness to allow the Jews, known as Falashas, to leave his country and asked Reagan to seek his permission "for the immediate resumption of the airlift."

Noting the reported deaths of "over 2,000 Falashas" in Sudanese refugee camps, the letter said: "Tragically, the survival of these people is in jeopardy, and they are at special risk."

The 700 Falashas (overseas sources reported only 500) airlifted Friday had been stranded in the Sudan after discovery of an earlier Israeli-mounted airlift that brought out 7,800 others. That was terminated in early January because of a leak in the Israeli press about the secret operation.

An aide to Cranston said the effort to focus Reagan's attention on the plight of the remaining Falashas had been "all hush-hush" to avoid another premature disclosure.

Yesterday, Cranston discussed the behind-the-scenes maneuvering by him and Sen. Alfonse M. D'Amato (R-N.Y.) to enlist support of the entire Senate in their effort for a U.S. airlift.

Cranston said that the letter had been written and signed Feb. 20-21 and that he cannot recall another issue on which all senators agreed so quickly to sign a letter to the president.

"It was pretty unprecedented," Cranston said. "Nobody had any hesitation."

Because of the rush to collect signatures, six duplicate names were attached to the letter.

"The astounding story is that the Senate can keep a secret," he said.

After the letter was sent, Cranston said, Reagan and Vice President Bush assured him that they were focusing attention on the Falashas' plight. Bush reportedly negotiated the airlift with Nimeri during his visit to the Sudan this month.

Cranston said that "a lot of credit" was due to Bush for his successful negotiations with Nimeri.

It was not immediately clear what role the Senate letter played in persuading the administration to take action. Cranston said he understood that low-level discussion of such an airlift had been under way at the State Department, but that the letter "helped focus attention" at the highest levels of government.

Cranston said he had no prior knowledge of the U.S. plan to send 10 C130 Hercules transport planes to the Sudan last Friday to airlift an expected 1,500 Falashas. Only about 700 were at the Tawara refugee camp outside Gedaref, where the planes landed.

Cranston and officials of the small American Association for Ethiopian Jews speculated that the other 800 had died in the camp, returned to Ethiopia, or made their way separately to Israel.