Abstract: Solving the oblique incident seismic wave field under irregular terrain conditions is challenging, and previous methods are limited in calculation accuracy and application range. A ground motion input method was proposed for irregular terrains based on wave field separation by combining analytical derivation and finite element simulation. Seismic P waves and SV waves are separated under different boundary conditions. When incident vertically, they are decomposed into free wave field and scattered wave field at the side boundary, and incident wave field and boundary outer field at the bottom boundary. For oblique incidence, the seismic wave at the opposite boundary of the input side is separated into incident wave field and outer boundary field. In this paper, the influence of local topographic conditions was fully considered, and the improved wave method was used to input nodal force conveniently. Besides, the vibration responses of regular and irregular terrains under different incidence angles were analyzed. The results show that the method has high computational accuracy and efficiency under multiple topographic conditions and can be applied to complex site conditions. Moreover, the incident angle and local site conditions are found to have significant effects on surface displacement response. The study can provide an effective way for response analysis under irregular terrain conditions.