Abstract: Tianshengqiao Ⅰ, Guangzhao, Longtan and Baise reservoirs are the backbone reservoir projects for water allocation in the Xijiang River Basin. They are used as objects to study the water storage capacity of the Xijiang backbone reservoirs at the end of flood season to ensure the water supply security of the basin. This paper uses the typical water inflow process with runoff frequencies of 75%, 90%, and 95% respectively, and based on the aggregation-decomposition water supply regulation model of the Xijiang reservoir group to calculate the comprehensive water shortage of each reservoir, using the reverse order recursive algorithm of water balance to obtain the phased water storage volume of each reservoir under the corresponding water inflow frequency. The results show that in order to ensure the safety of water supply in the basin during the dry season, the effective water storage capacity of Xijiang backbone reservoirs at the end of the flood season should be at least 6.3 billion m³, 8.3 billion m³, and 10 billion m³ respectively in the runoff frequencies of 75%, 90%, and 95%.