A large earthquake usually consists of mainshock and aftershock. When the epicenter of the earthquake is close to the ground surface, the oblique incidence of mainshock and aftershock often occur near the fault area. Therefore, it is necessary to consider the case of oblique incidence of seismic waves at multiple angles instead of vertical incidence only. Three mainshock-aftershock sequences measured near the fault area were selected to derive the equivalent node-force formula of ground motion P wave oblique incidence interface, and then the feasibility of the formula was verified. Taking Koyna Dam as the research object, the seismic response of the dam at the incidence angle of 0°-90° was studied under two working conditions, namely, single mainshock sequence, mainshock-aftershock sequence. Three indices of displacement, damage index and damage dissipation energy were used to analyze the damage of the dam body after the earthquake, and the local damage index was established to quantify the damage to the dam. The results show that when the incidence angle varies in the range of 15°-75°, the seismic response of the dam is greater than that of the vertical incidence under the two working conditions. Under all three performance indices, the seismic response of the mainshock and aftershock sequence is greater than that of the single mainshock.