By Agence France-Presse
The blast in the small hours of Wednesday in the city of Zhangjiakou, some 200 kilometres (125 miles) northwest of Beijing, also injured 22 people.
"Initial investigations have shown that the explosion was caused by a gas leak from the Hebei Shenghua Chemical Co. of ChemChina," Zhangjiakou mayor Wu Weidong told reporters.
"Vinyl chloride spread to the public road outside the factory and was ignited by an open flame."
Authorities previously reported that a truck carrying combustible chemicals had blown up while entering a factory, igniting other nearby vehicles, but the cause of the explosion had not been known until now.
Thirty-eight trucks and 12 cars were also damaged in the explosion, which eyewitnesses say created a huge fireball that engulfed vehicles on an entire stretch of road.
The explosion occurred in the city's Qiaodong district some 45 minutes away from Chongli, which will host some of the mountain sport competitions during the 2022 Winter Games in Beijing.
Industrial accidents are common in China, where safety regulations are often poorly enforced.
In 2015, giant chemical blasts in a container storage facility killed at least 165 people in the northern port city of Tianjin.