Like many vehicles in Chengdu, the bus ran on natural gas rather than gasoline.
Officials said more than 10 passengers managed to escape the burning vehicle.
Pictures posted on internet forums showed the bus in flames.
One badly burned woman was seen lying on the road with a man, his clothes burnt off, standing near her.
A report on a website run by the Sichuan provincial government said the cause of the blaze was still being investigated.