The nomination of Winston Lord to be ambassador to China is stalled on the Senate floor, caught first in a general Democratic hold on nominations and now blocked by a new filibuster threat by conservative Republicans.

Led by Sen. Jesse Helms (R-N.C.), conservatives have written to President Reagan and Senate Majority Leader Robert J. Dole (R-Kan.), opposing Lord because they say he supports continued aid to family planning organizations working in China. That nation's family-planning policies reportedly include compulsory abortions and tolerate female infanticide.

Helms said yesterday he is awaiting a letter from the White House or from the Agency for International Development that would ease his concerns before allowing Lord's nomination to come to a vote.

"This is the only lever I have to prompt the administration to be specific on its position," Helms said in an interview. "I get lots of double-talk from AID."

Although the Republicans have not said so publicly, they are also wary of Lord's background as head of the Council on Foreign Relations, the moderate foreign policy establishment, and as chief aide to former secretary of state Henry A. Kissinger. Lord, 47, was nominated in July to succeed Arthur W. Hummel Jr., and administration officials had openly expressed hope that Lord would be on the job in time for Vice President Bush's visit to China, which begins Thursday.

"It would be helpful for him to meet the Chinese officials as part of the vice president's visit," said Bush spokesman Marlin Fitzwater.

Senate Minority Leader Robert C. Byrd Jr. (D-W.Va.) has been blocking action on executive branch nominations since Congress returned to work last month to protest the fact that Reagan made several appointments during the congressional recess. However, the Democrats agreed last Thursday to allow Lord's nomination to proceed.

Helms delayed Lord's nomination in the Foreign Relations Committee until last week, when it was approved on a 16-to-1 vote, with Helms the lone dissenter. Helms then joined four other senators in writing to Reagan to oppose the appointment. The others were Republican Sens. William L. Armstrong of Colorado, Orrin G. Hatch of Utah, and Chic Hecht and Paul Laxalt of Nevada.

"We do not think Mr. Lord's testimony before the Senate Foreign Relations Committee demonstrates that he understands your attitude and policies with regard to abortion and foreign assistance," they said.

Lord had testified that he agrees with AID Administrator M. Peter McPherson, who has said U.S. law would allow continued aid to the United Nations Fund for Population Activities, provided the fund withdrew from any management role in the Chinese population effort.

The conservatives wrote, "The abuse is the inhuman Chinese program itself." Conservatives have argued that the UNFPA should be required to withdraw entirely from China before receiving U.S. funds. "It is inappropriate for Mr. Lord to go to China so long as he supports the interpretation of the law given by Mr. McPherson," the letter said.

In a letter to Dole last week, conservative senators asked that Lord's nomination not be brought to the floor, saying "it would be an occasion for extended debate on China's population policies."