The invention discloses a method for determining an effective utilization coefficient of a creep distance of a composite insulator. At present, the method for the effective utilization coefficient of the creep distance of the composite insulator does not consider a non-uniform characteristic of natural dirt retention of a control insulator and has a certain limitation. According to the method, by analyzing characteristics of equivalent salt deposit density and equivalent non-soluble deposit density of the upper and lower surfaces, subjected to the natural dirt retention, of the composite insulator and the control insulator, a dirt retention ratio of the composite insulator to the control insulator subjected to the natural dirt retention is obtained; according to a manual withstand pollution test result, flashover voltage characteristics of the composite insulator and the control insulator are obtained; and according to corresponding flashover voltages under different dirt retention conditions, the effective utilization coefficient of the creep distance of the composite insulator is obtained. According to the method, the influence of salt deposit density, non-soluble deposit density and dirt retention non-uniformity of the natural dirt retention of the insulator on the flashover voltage is considered, so that the effective utilization coefficient of the creep distance is ensured to be correctly calculated.