[Objective] The formation mechanism of rainstorm-induced debris flow in Yushu Plateau of Qinghai Province is researched. Which was expected to provide a basis for the prevention and control of debris flow disaster and reduction of the risk mitigation. [Methods] Through field survey, Lalong debris flow of Chengduo County in Yushu state was selected as an example, and the stimulus factors and mechanism were analyzed. [Results] The main channel length of the debris flow is about 3 km, the catchment area is 5.25 km², the longitudinal gradient is 212‰, and the height difference is about 600 m. The volume of the landslide of the source area is about 6.50×10⁴ m³, the volume of the debris flow is about 8.5×10⁴ m³; and the volume of the material in the channel is 2.0×10⁴ m³. [Conclusion] The process of rainstorm-induced debris flow in the study area can be divided into 4 stages: landslide-source formation, particle material starting up, gully erosion and debris flow initiation.