Abstract: Based on the dynamic differential equation of beam, the dynamic interaction factor under lateral seismic excitation considering the effect of axial load is analyzed in this paper. It is found from the analysis under special situation that the large axial load had obvious effect on dynamic interaction factor of group pile. The affected elements are analyzed in comparison with previous result, which mainly are dimensionless frequency, the elastic modulus ratio of pile to soil, and the distance between two piles etc. The numerical results showed that the method is more rigorous to be used to calculate the dynamic interaction factor.