Trade between the United States and China reached $8.1 billion last year and can grow steadily if both sides are careful, U.S. Ambassador to China Winston Lord said yesterday in his first major speech since taking office last November.

Lord told the National Council for U.S.-China Trade that the apparent slowing in China's reform process is only a period of consolidation in which the leaders are addressing problems of rapid change.

"The leaders stress that the reforms and the openings are irreversible, that the momentum will resume in 1987," he said. "If it can maintain political stability, China will become stronger with or without U.S. assistance."

The two nations are in a "relatively quiet phase of sound relations" after 13 years of trading, Lord said. The $8.1 billion figure was 25 percent higher than in 1984 and $1 billion more than was predicted, and resulted in a $300 million balance in China's favor and $1.4 billion in U.S. investment in China.

The trade nearly was triple the $2.8 billion U.S. trade with the Soviet Union and about double U.S. trade with India, according to Commerce Department figures. While grain sales plummeted as China's ability to feed itself rose, sales of U.S. machinery for coal cutting, mining and drilling wells boomed, as did exports of office equipment, computers, commercial aircraft and power generators. In turn, U.S. imports involved clothing and accessories, toys, games, sporting goods and crude oil, the figures showed.

"I come not to celebrate achievement but to censure complacency," Lord told the 400-member association of U.S. corporations doing business with China. He urged them to oppose protectionist trade legislation pending in Congress because "if China cannot sell to America, America will not sell to China."

In return, China should realize "there is bound to be some correlation between China's sharing of geopolitical perspectives and our sharing of advanced technology, especially military," Lord said.

The Chinese "still do not comprehend the American system" of government and economics, he continued. American businessmen suffer in China from "high costs, tight foreign-exchange controls, limited access to the Chinese market, lack of qualified local personnel and unpredictability" of government regulations, Lord said.

Lord noted that the Chinese often blame world troubles on U.S. and Soviet "contention . . . for hegemony" worldwide. "We do not appreciate being confused with the Soviet Union," he said. China is moving to ease its relations with Moscow in the belief that Moscow "looks decidedly less formidable" at the moment.

"We do not seek an alliance with China any more than China seeks one with us," Lord said. "Nor do we wish to block the improvement of relations between Moscow and Peking. Conflict between the two giants would be dangerous. Cooperation between them will be limited" for historical reasons.