Abstract: Using the multi-transmitting boundary formula to solve scattering problems such as seismic response analysis of soil-structure interactive system, and instability of the multi-transmitting boundary formula is an important aspect which is needed to be studied and focused on. The studies on the drift instability mechanism of multi-transmitting boundary formula show that when the total wave field is separated into incident wave field and scattered wave field for wave motion input and applying the multi-transmitting boundary condition, there is difference of incident wave field between the analytical solution in continuous medium and the real one in discrete grids, which may result in drift instability. For this problem, an improvement measure is proposed, that is , the numerical solution of incident wave is obtained by establishing the computation model for the boundary region with the multi-transmitting boundary formula, which is used in wave motion input process. The numerical experiments show that the proposed measure can effectively eliminate the drift instability in the calculation and give more reasonable results.