Abstract: Big storage and long calculation time requirements are main obstacles for the development of moment method. Therefore, how to efficiently use geometric symmetric feature of scattering structures to reduce computer storage and calculation time for solving the scattering problems is significant in theory and application. In this paper, for the scattering structure with single axial symmetry, a new approach based on moment method is proposed by introducing imaginary decomposition. With this approach, the dimensions of matrix can be reduced to half of the ones in direct calculation, and three-quarter calculation time can be saved. Meanwhile, the formula for decomposition and the decomposition theory are also presented.