"The 55-seat bus carrying 50 people fell off a 10-metre-plus-high cliff after crashing into a sports utility vehicle and a pick-up truck," state-run Xinhua news agency reported.
Another five people were in the other vehicles in the accident which happened at around 4:25 pm (0825 GMT) in Nyemo County, west of the capital Lhasa, in the Tibet Autonomous Region.
The bus passengers were mainly tourists from regions including Anhui, Shanghai, Shandong and Hebei.
Police have detained managers of the Feixiang Travel Agency and the Shengdi Vehicle Tour Company blamed for the accident.