This paper presents a theoretical dynamic model based on the Hampton criterion and Mayr's equation for predicting the critical values of flashover on the surface of nonuniformly polluted cap and pin porcelain insulators. The nonuniform pollution layer on the surface of the insulator, the number of disk insulator units, and the geometry of the insulator such as diameter and ribs are the important factors used for prediction. Two types of pollution layer configuration, linear and graded, are introduced as nonuniform pollution layers. The influences of the nonuniform pollution layer on critical flashover voltage are investigated. Furthermore, a random value is added to the surface discharge length during propagation in order to consider the effects of the wind and thermal convection on surface discharge length. The specifications of a real porcelain disk insulator are used in the model. Finally, the estimated results of the proposed model are compared with the measured data of the tests in order to validate the proposed model.