[Background] The upper reaches of Jinsha River is a geological disaster prone area, and the geological environment evolution is relatively complex. [Methods] To study the relationship among the source of deposits after the break of dammed lake, the materials in the dam-body and sedimentary environment, taking the dam-break deposits from the dammed lake caused by Xuelongnang ancient landslide in the upper Jinsha River as the main research object, through field survey sampling and indoor sieving test of particle size, the particle sizes were tested, the granularity characteristics in the upstream, middle, and downstream section of the deposits were studied, and the composition of grain size and the size distribution curve of the deposits were analyzed in detail. [Results] The results showed that the distribution curves of grain sizes in the dam-break deposits for the upstream, middle and the downstream sections were unimodal, multimodal and doublet respectively. The composition of the grain size in the upstream section mainly was gravel, the hydrodynamic intensity was very strong, and it was affected by the single stable hydrodynamic condition. In the middle section, the main ones were fine sand and extremely fine sand, hydrodynamic intensity weakened mainly in the lower reaches. Compared with middle section, also main sizes in downstream were also fine and extremely fine, and the proportion of them increased; moreover, hydrodynamic intensity was even weaker in general. Regarding the relationship between deposits and materials of dam-body, the gravels were from the material of dam-body, and the fine particles might be partly from the dam-body, and partly from the lacustrine sediment in the upper reaches before the break of dammed lake. The particle size presented the trend of obvious becoming thinner from the upstream to the downstream, and the average particle size changed from coarse to fine. The sorting feature of dam-break deposits was poorer and poor. These characteristics reflected the gradual weakening of hydrodynamic intensity from upstream to downstream of dam-break deposits. Contrast to the characteristics of the wind sand dune-sea (lake) beach sand-river sand-glacial and alluvial fan, etc. studied previously, the size distribution of dam-break deposits in the upstream of Jinsha River had high similarity with the grain size distribution curve of the river deposits. [Conclusions] Therefore, to reveal the granularity characteristics and change pattern of the deposits, studying the relationships among the sources of deposits from break of dammed lake, the material composition of dam-body and sedimentary environment is of great significance, which may provide reference value for the management of soil and water conservation in the upper reaches of Jinsha River in the future.