Pictures said to show a fireball in Dongying, in the eastern province of Shandong, circulated on social media.
The explosion occurred on Monday night at the Diao Kou Xiang Bin Yuan Chemical Co., according to a news website run by the local government.
The fire was extinguished about five hours later and "people in charge of the company" were detained by police, the site said.
China's official Xinhua news agency put the toll at five dead.
Last month's disaster sparked widespread outrage over alleged safety violations and possible official collusion, and fears of pollutants contaminating the air and water of Tianjin, home to about 15 million people.
Another explosion at a chemical plant in Shandong killed one person and injured nine others on August 23.