To explore the related issues in emergency water diversion of Xingou River Project, and to evaluate the water diversion effect, the pattern of influences on water diversion caused by variation of water quality in the basin under Xingou River Project was analysed based on the method of comprehensive pollution index, with the water quality improvement index(I) and the category change index(G) introduced. Results showed that during the water diversion period, the diversion ratio of the west main branch of Xingou River is 0.30~0.59. Affected by the diversion of the west main branch of Xingou River, the daily average water inflow of the east main branch of Xingou River into Lake Taihu accounts for 9%~22% of the daily average water quantity from the Yangtze River. Three days after the continuous water diversion through Xizhihugang-North Project, water from the Yangtze River has an impact on the east main branch of Xingou River, south of the Beijing-Hangzhou Grand Canal. The effect of water quality improvement in main stream is related to the background water quality of each section, as well as the distance between each section and the water source. For those sections with higher initial value of the comprehensive pollution index, the I_max index tends to be larger. For the sections with similar background water quality, the I index of those sections close to the water source tends to be larger.