Imports of common alloy aluminium sheet from China were valued at an estimated $600 million in 2016, the department said in a statement.
A final determination in the countervailing duty investigation is scheduled to be announced on Aug. 30, it said.
The Aluminum Association, a U.S. trade group, welcomed the decision. "This is an important first step to begin restoring a level playing field for U.S. aluminium sheet production," the group's president, Heidi Brock, said in a statement.
The department's preliminary finding comes as President Donald Trump has threatened to slap tariffs on some $150 billion of Chinese goods to try to force changes in China's industrial policies.
The Commerce Department launched the investigation of imports of Chinese aluminium alloy sheet in November, the first U.S.-initiated anti-subsidy and anti-dumping probes in decades.
Washington's seldom-used tactic is aimed at accelerating the imposition of duties against what are determined to be unfairly subsidized and dumped products. U.S. companies and industries claiming injury from imports would normally first ask the Commerce Department to open such probes, but government-initiated cases skip that step.
The department said it calculated a preliminary subsidy rate of 31.20 percent for Yong Jie New Material Co Ltd; a rate of 34.99 percent for Henan Mingtai Industrial Co Ltd and Zhengzhou Mingtai Industry Co; and a rate of 113.30 percent for Chalco Ruimin Co Ltd and Chalco-SWA Cold Rolling Co Ltd.
The preliminary subsidy rate for all-other Chinese producers and exporters is 33.10 percent, the department said.
(Reporting by Eric Beech and David Lawder; Editing by Mohammad Zargham and Sandra Maler)