A typical small hydropower plant “Cangpinghe” was investigated monthly for studying the impacts of small hydropower plants on macroinvertebrate habitat and the ecological water requirement of Xiangxi River. The weighted usable widths (WUW) were calculated for three dominant orders of the macroinvertebrate (Ephemeroptera, Plecoptera and Trichoptera) at each cross section and some comparisons were conducted among cross sections. The results show that the dam′s diversion caused part of the downstream channel dried up and the WUW of the three orders of the macroinvertebrates declined significantly between the upstream channel and the downstream channel. According to the relationship between the WUW and the river discharge, it was calculated that the minimum ecological water requirement for the main group of the macroinvertebrates in Xiangxi River was 3.8 m3/s.