In a significant sign of a new explosion in trade and contact between the United States and China. American businessmen have completed their most sucessful Canton trade fair ever with deals totaling an estimated $145 million.

John Kamm, a veteran American trade analyst who has been in contact with both U.S. businessmen and Chinese officials, said the estimated $83 million in U.S. sales and $62 million in purchases doubled the previous record for American business here. Some traders said the figures reflected growing Chinese interest in invading the troubled U.S. clothing market before stiff import quotas are imposed.

Po Shu-sen. deputy secretary general for the fair, China's principal foreign trade exhibition, said overall business at the month-long autumn session ending today exceeded all 43 previous fairs held since 1957 in this southern Chinese port. Western analysts estimated that the Chinese earned about $1 billion in needed foreign exchange here while buying about [WORD ILLEGIBLE] million in foreign goods

Kamm, representing the Washington-based National Council for U.S.-China Trade, called the U.S. business figures "incredible." He said the surprisingly keen interest shown by the Chinese in the American market could push total U.S.-China trade this year back to 1974 level of about $1 billion.

Trade between the two countries fell off after 1974 when the Chinese ran out of ready cash and became preoccupied with their internal political struggles. The business at this fair, however, plus an unprecedented $500 million deal signed last week for construction of several American-style hotels in China, indicate a renewed flow of money and goods between the two countries despite their failure to establish full diplomatic relations.

Businessmen here said they were struck by the flexibility and capitalist-style aggressiveness shown by the Chinese government officials who negotiated with foreign traders here.

During an interview yesterday, Deputy General Secretary Po showed particular interest in the product pay-back schemes being suggested by several American businessmen. Under such arrangements, cheap Chinese labor would assemble U.S.-designed goods with U.S. equipment on Chinese soil and pay off the American investors with some of the finished goods.

Several Hong Kong businessmen have been allowed to initiate such schemes, and at least three Americans have had preliminary approval, foreign businessmen here said.

Some traders and analysts here said they noticed Chinese factories and trade offices actually competing with each other to line up lucrative foreign contracts, a surprising development in this state-planned economy. Kamm said he thought some Chinese officials were being given bonuses by their government for particularly profitable sales or cut-rate purchases. Such incentives have previously been forbidden but new administration in Peking is trying to revive the economy through morale-boosting measures that were frowned on before the death of Chairman Mao Tse-tung two years ago.

Kamm estimated that the Chinese spent $80 million, almost the total of their purchases from U.S. traders, on American chemical products. Much of that consisted of polyester chips useful in the manufacture of synthetic textile goods. Kamm said Chinese officials indicated they sold about $20 million in textiles, mostly garments and piece goods, to U.S. buyers at the fair, a significant jump from the approximately $15 million in textile sales to Americans at both the autumn 1977 and spring 1978 fairs.

Other Chinese purchases here included plastics, specialized chemicals for China's budding petroleum industry, and herbicides and fertilizers for its sluggish farm industry. Kamm said traders noticed the beginnings of significant Chinese purchases of American medical equipment - including X-ray machines and pacemakers - as China upgrades its health system. In addition to textiles, Americans purchased about $10 million in minerals and metals such as tungsten, and barite, plus significant quantities of foodstuffs, animal byproducts such as feathers and pig bristles, baskets and fireworks.

Po credited the increased business to the partial revival of Chinese Industry, although he noted that supplies of some goods were still low and freigners still complained that deliveries were sometimes late. Many of the American textile purchases will not show up in the 1978 trade totals because they are not scheduled for delivery until late 1979.

Po said he was grateful to U.S. and other foreign businessmen for suggestions on how to improve Chinese sales. One American packaging expert told the Chinese their "Peony" brand name for transistor radios was too old-fashioned and would discourage U.S. consumers. The expert pointed out that another brand name for some food products. "White Elephant" has derogatory connotations in the United States.

Po said a record 20,000 businessman attended the fair and plus another 20,000 tourists and visitors came during the month of trading. He said about 110 countries and regions were represented.

Kamm estimated about 500 U.S. businessman representing 320 companies visited the fair. Kamm had earlier predicted no more than $75 million in U.S. business during the fair, principally because of the growing numbers of deals that are being hatched in Peking instead of Canton.

American sellers also profited from the decline in value of the dollar against the Japanese yen. Chinese in several instances told Japanese traders that their prices would have to come down. In the meantime the Chinese turned to the United States for chemicals they had been buying from Tokyo.