Abstract: On July 16, 2018, a large debris flow occurred in the Yunmeng Mountain region of Huairou district, Beijing. The event seriously destroyed two downstream highways of G111 and S310, forced an emergent transfer of 506 people in adjacent 4 towns. This paper interprets the high-resolution satellite images of pre-and post-event and in-situ investigation, and studies the geomorphologic and geologic characteristics of two typical debris flow gullies and the sources of solid materials, analyzes the rainfall condition of debris flow initiation, calculates the major parameters such as bulk density, flow velocity and total amount of the primary process, and deduces a quantitative relationship between the initiation of debris flow and the rainfall intensity on the basis of stability analysis of gully blocks. The results reveal that "7·16" event belongs to dilute debris flow triggered by the short-time heavy rainfall. Under continuous rainfall for 2 hours, the critical rainfall intensity that could trigger debris flow is no less than 8~15 mm·h^-1. The solid materials of debris flow are mainly sourced from the gully-bed erosion and the lateral erosion. It suggests that debris flow of same type and scale can be very likely to take place under similar rainfall conditions. The study can provide a useful reference for future hazard prevention and mitigation.