Aperture coupling is the most important way of electronic interference. In the existing literatures, the method of calculating the average electrical susceptibilities of arbitrary shaped apertures is based on hypergeometric theory. Although this method can accurately calculate the average electric polarization density of small holes in any shape, the hypergeometric function is too complex for the engineers of non-mathematics. Therefore, the method of calculating the average electric polarization density of small holes is seldom used in the field of electromagnetic field engineering. In order to solve this problem, a new analytical formula is proposed in this paper, which greatly simplifies the calculation without loss of precision, and can greatly improve the efficiency of the solution of the small hole coupling problem. The core idea of this method is to convert the calculation of the average electric polarization density of a small hole to the arithmetic average of the average electrical polarization rate density of numerous small circular holes, and then deduce an analytic formula of the line integral along the edge of a small hole. The average electrical polarization rate density.