The recommended method for calculating the number of lightning strikes to overhead lines in the technical specification is studied, and it is pointed out that in the method, there is mismatch between the collection area and the amplitude of lightning current corresponding to the lightning ground flash density. To solve the problem, the method for calculating the number of lightning strikes using integration is introduced through a example. Comparison of calculated results between the recommendation and integration methods illustrates the elaboration of the integration method and the rationality of the results. The integration method is based on the electrical geometric model to deduce the computational formula of overheadline collection width related to the amplitude of lightning current, and to determine the ground flash density corresponding to any lightning current amplitude by means of the polynomial fitting of frequency distribution regularities of lightning current amplitude. The product of collection area and lightning ground flash density corresponding to any lightning current amplitude is the annual mean number of lightning strikes corresponding to the lightning current amplitude. Integrating the computed number of lightning strikes within the distribution range of lightning current amplitude for this region, the annual mean number of lightning strikes considering the distribution characteristics of lightning current amplitude can be obtained.