The factory in southern China's Guangxi province exploded on Friday afternoon, the official Xinhua news agency said, without citing a cause.
Local police have detained the manager and the legal representative of the factory, the report said.
China has seen several firework-related disasters in recent years.
Eleven people died in February after a lorry carrying fireworks exploded and caused a highway bridge to collapse, local officials said.
Explosions at two different fireworks factories in China last year left a total of 14 people dead and 13 injured, Xinhua reported.
China's vast fireworks industry is notorious for lax safety standards. Exported Chinese fireworks often do not meet safety standards at their destination countries.