This standard describes the test method for determining the average pore size of separation membranes by gas permeation method. It is suitable for flat membranes and hollow fiber membranes with pore sizes of 0.01 μm to 0.2 μm. Miscellaneous form separation membranes can be referred to for implementation. It is suitable for using non-coacervation gases such as nitrogen as test gases. Through standardized test methods, this standard provides an important basis for the quality control of separation membrane materials. It is widely used in the application evaluation of membrane technology in water treatment, gas separation and other fields.