The explosion was heard at about 7:40 AM in the Lanjiang village of Meitan County, where people were believed to be making fireworks illegally, the state-run Xinhua news agency reported.
Four were killed on the spot and two others were sent to hospital.
While one of the wounded later died, the other is in stable condition.
This is a second major explosion at a fireworks factory in China in recent days.
Last month, 12 people were killed and 33 injured in a blast at a firework plant in central China's Hunan Province.