The fire broke out at 1 pm (local time) when the bus was passing by a primary school at Guiyang City of Guizhou Province, provincial information department said.
The number of injured has risen to 35 from 30, it said.
The injured were admitted to four different hospitals, state news agency Xinhua reported.
The driver of the bus said there were about 50 people on board the bus when the fire broke out. "I opened the door at once out of instinct, so that people could get off as soon as possible," he said.
The information department said since January last year all 1,850 buses operating in the province had started using liquefied natural gas instead of gasoline for green purposes.