To study the influence of obliquely incident seismic waves on the vulnerability of powerhouse at dam toe, the design seismic component consistent with measured ground motion was first obtained on the surface through the superposition of obliquely incident SV-wave field and P-wave field. Meanwhile, in order to consider the dynamic damage of the powerhouse, a program of structural vulnerability analysis considering the system of obliquely incident waves was compiled, in which a damage constitutive model of concrete was embedded. Finally, 26 near-field ground motions were selected from the PEER database and modulated according to the peak ground acceleration (PGA). Taking a powerhouse at dam toe in southwest China as an example, the vulnerability curves of its upstream and downstream walls under the action of obliquely incident system with different intensities were calculated with the incremental dynamic analysis method. Through comparison, it is found that the failure probability of seismic vulnerability curves under vertically incident waves is relatively higher than that under obliquely incident waves, with the maximum difference of 26%. The failure probability of downstream wall is higher than that of upstream wall, and the maximum difference is up to 19%. Thus, the seismic design of powerhouse at dam toe should consider the effect of obliquely incident seismic wave, and more attention should be paid to the seismic safety of downstream wall.