Abstract: The Tianmo Gully debris flow in southeastern Tibet is a typical one, which broke out frequently in 2007, 2010 and 2018, causing harm to local residents and the Sichuan-Tibet highway for many times. The source materials distribution and types before and after debris flow events were drawn by means of multi-stage remote sensing interpretation, field investigation and field observation. GIS statistics and profile mapping were used to compare the changes of debris flow source materials area in plane and profile in different periods in the gully for the study of the dynamic evolution of the source materials. The source materials types of debris flow in Tianmo Gully are mainly moraine type, collapse type and erosion type. On the plane, moraine type source materials has the largest area, collapse type source materials is the most concentrated, and erosion type source materials extend the longest distance. On the profile, the source materials of the main channel varies most actively from 13° to 23°, among which, 17° to 23° is the starting site of debris flow erosion type, and 13° to 17° is the most intense site of debris flow erosion. The dynamic evolution of the source materials in Tianmo Gully is characterized by the "convergence and rapid melting" effect of avalanche accumulations year by year, and the instability of loose materials at the bottom and both sides of the channel. The avalanche accumulates in the main gully and extends downstream 1800 m from October to June of the next year. From July to September, the avalanche deposits are rapidly melted, releasing a large number of loose materials bound in the avalanche deposits and making the runoff in the gully increase sharply. The surge of runoff erodes the old moraines on both sides of the main ditch and converts them into debris flow sources. Research on the provenance evolution characteristics of debris flow in Tianmo Gully can reveal the reasons for the frequent occurrence of debris flow in Palongzangpo Basin, and provide reference for the early warning and disaster prevention and mitigation of debris flow in this region.