Sparks from a pipe being welded at Qilu Tianhe Huishi Pharmaceutical Co. in eastern Shandong province caused a substance to catch fire and give off the smoke, state news agency Xinhua reported.
Twelve rescue workers were hurt but were not in life-threatening condition, it added.
Qilu Tianhe has invested 1 billion yuan ($150 million) in its massive pharmaceutical plant covering 280,000 square meters in the city of Jinan.
It exports its medicines that fight infections, cancer, and cardiovascular problems to Europe, North and South America, and the Middle East, the company's website said.
The accident is under investigation, according to Xinhua.
Deadly industrial accidents are common in China, where safety regulations are often poorly enforced.