Abstract: Group targets are widely present in electromagnetic engineering, and the development of efficient and accurate simulation methods for them is of great significance. Considering the characteristic that the unit arrangement of dynamic group targets may vary at different times, this paper first proposes a fast multipole algorithm (FMA) based on unit grouping. The method uses the theory of characteristic mode (TCM) to reduce the unknowns of group targets and uses a pre-storage strategy to achieve efficient and accurate analysis of dynamic group targets; Then, in order to consider the influence of sea background on electromagnetic scattering of group targets, a hybrid method of high and low frequency is proposed by combining the fast analysis method based on TCM of dynamic group targets proposed above with the high-frequency physical optics (PO) method. Numerical examples demonstrate that the proposed hybrid high-low frequency method can achieve efficient electromagnetic characteristic analysis of dynamic group targets and the sea background as a whole while ensuring high computational accuracy. This approach thereby achieves a reasonable balance between computational efficiency and precision.