Abstract: Biofilms attach to surfaces of bed sediments contain a large number of microorganisms, which play an important role in decomposition of organic matter and carbon and nitrogen cycling in river channels.Nevertheless, the specific composition of bacterial communities in biofilms on bed sediments with different particle sizes remains unclear.Based on an examination of four sets of biofilm samples collected from the Shiting River, we studied the composition of bacterial communities in the biofilms on the sediments with different particle sizes using High-throughput sequencing.The results showed that there were significant differences between the sediment less than2mm and the sediment larger than2mm, with notable disparity in species abundance at phylum level and genus level.Furthermore, the finest sediment(<2mm)exhibited the highest Shannon and Chao indices, indicating the highest diversity.In addition, the diversity was higher at upstream of the weir compared to downstream.This was likely due to the riverbed erosion occurring downstream of the weir, which was unfavourable for growth of the epilithic bacteria.ANOSIM tests demonstrated that the spatial location exerted a more pronounced influence on the composition of bacterial communities in epilithic biofilms than sediment particle size in the Shiting River.The research results can provide a reference for the ecological restoration of mountainous rivers.