An American engineering firm won an $80 million contract today to help build a huge, ultramodern steel plant near Shanghai, the first U.S.-China trade deal to be financed by the U.S. Export Import Bank.

The contract, signed in Shanghai by Wean United Inc. of Pittsburgh, is expected to encourage several other U.S. businesses seeking low-cost financing for projects in China. The U.S. government agency will provide 85 percent of the financing under a preliminary commitment arranged by Wean before today's contract was awarded. Chinese officials have pointed out to American businessmen in the past that they have suffered in competition with Japanese and European companies because of their inability to get government backing to lower the borrowing costs to Peking.

The new contract provides a significant American presence in the construction of the $5 billion Baoshan steel plant, scheduled to be one of the largest and most modern in the world. "There are not too many places left in the world where people are building new steel plants from scratch," one U.S. official said.

The American firm will handle part of the construction of the $500 million cold rolling mill in the plant as a subcontractor to a German firm, Schleoemann-Sieag of Dusseldorf. Austrian and Japanese firms also are involved in building the mill, which will produce 2.1 million tons of steel a year after its scheduled completion in 1985. Nippon Steel is involved in other portions of the Baoshan project, a key to China's plans for modernizing its industry.

Wean was awarded the contract by the China National Technology Import Corp. About 50 Americans will be working in China on the construction, which will begin immediately, company officials said.