Based on the principle of salt equilibrium in irrigation districts, the salt equilibrium of Weigan river irrigation district is analyzed by using measured data. The factors of affecting critical ratio of irrigation to drainage and the method of determining critical ratio of irrigation to drainage are also discussed profoundly. The results show that: (1) The conception of the ratio of irrigation to drainage is brief, embodies the characteristics of water conservation construction and irrigation agriculture, which can be used as the criterion of estimating the dynamics of moisture salt movement in plain oasis irrigation distract in arid area,and can also used as the scientific basis of management of irrigation and drainage ,planning and designing.(2)Critical ratio of irrigation to drainage is related to the irrigation water volume, drainage water volume ,salt content of irrigation water and drainage water and so on. The critical ratio of irrigation to drainage for Weigan river, Kuche county , Xinhe county, Shaya county are 10.8,6.55,20.99,21.48, respectively. If there is no special topography, the critical ratio of irrigation to drainage in oasis irrigation distract in arid area is about 21. (3) The seepage is necessary to leach salt, carrying out calculating of water balance, making irrigation schedule and managing the irrigation and drainage .At the same time, the drainage systems need improving to have the irrigation district operated under the condition that actual ratio of irrigation to drainage is less then critical ratio of irrigation to drainage.