The purpose of this study is to analyze the water supply guarantee for the construction of large-scale irrigation area. Based on the existing soil and water foundation conditions in the Nanxiong Basin irrigation area and the results of the third national land survey，the water balance analysis is conducted for the large-scale irrigation area of Nanxiong Basin. The main conclusions are as follows：1) the designed irrigation area of the Nanxiong Basin irrigation area after completion can reach 473 100 acres；2) the Nanxiong Basin irrigation area has formed preliminary water system connectivity，and its 13 zones can achieve joint scheduling of multiple water sources；3) the irrigation water demand of Nanxiong Basin irrigation in the dry years with the rainfall frequency of 90% is 247.141 2 million cubic meters，with an average annual urban and rural water supply of 46.172 5 million cubic meters，an average annual ecological water demand of 39.688 4 million cubic meters，and an average annual total inflow of 416.984 6 million cubic meters from the main water sources. The overall available water supply of the irrigation district can meet the water demand.