In this paper, a dynamic model to calculate the flashover voltage of polluted insulators is presented. This paper focuses mainly on two points: The criterion of arc propagation and the arc length in which the flashover occurs. For first point, the typical criteria of arc propagation are not regarded but also the main criterion of arc propagation is used. For the second point, a computer program is used so that the obtained results show that the flashover occurs when the arc bridges about 67% of the insulator length. To evaluate the proposed method, four sample systems are considered and the flashover voltage for different surface conductivities and ESDDs is obtained. The results based on the proposed method are compared with those of three proper methods. The accordance between the acquired results shows precision of the method.