Eleven were injured in the explosion on Thursday in a highway tunnel under construction between the cities of Dujiangyan and Wenchuan, state media reported on Friday.
Most of the victims were construction workers.
The exact number of casualties was unclear, with search and rescue operations still under way.
China is known for its poor workplace standards, as pressure to mine resources and build roads and other infrastructure to keep pace with the demands of the world's fastest-growing major economy leads to compromises on safety.
Its mining industry is the world's deadliest, claiming 6000 lives last year.
Li Yizhong, director of the State Administration of Work Safety Supervision, was to give the results of a government investigation into recent coal mine accidents later on Friday.
The International Labour Organisation estimated the number of fatal workplace accidents in China in 2001 at more than 90,000.