The blast in Hunan's capital city of Changsha, home to a hub for fireworks manufacturing, occurred on Monday local time, according to reports by CCTV and Xinhua.
Videos on Chinese internet platforms showed thick smoke billowing from a large site with collapsed buildings and debris strewn around the area. Reuters could not verify the footage.
Almost 500 firefighters, rescuers and medical personnel attended the scene, according to the South China Morning Post.
The blast happened at Huasheng Fireworks Manufacturing and Display Company, media said.
Xi called for a speedy investigation to determine the cause of the blast and strict accountability for the incident, Xinhua reported.
Last year, China exported $US1.14 billion ($A1.59 billion) worth of fireworks, more than two-thirds of global sales, data from the Observatory of Economic Complexity shows.
Xi also ordered authorities to strengthen risk screening and hazard control in key industries, enhance public safety and ensure the safety of people's lives and property.
Xi often issues "important instructions" to local officials after major accidents and disasters with fatalities.
Last week, he urged a nationwide upgrade in China's disaster response capacity.
Xi had also issued instructions after a blaze in Hong Kong in November at the Wang Fuk Court complex that killed 168 people.
With AP
