The one-dimensional and two-dimensional time-domain inversions of the bedrock incident waves cannot reflect the spatial arbitrariness of the incident azimuth and oblique incident angles，and the free field constructed based on these two methods might deviate is far from reality. It is necessary to carry out a three-dimensional time-domain inversion of the bedrock incident waves. Based on the wave function combination method，the free field of the ground surface control point is expressed by the time histories of the incident waves of P，SV and SH waves. A three-dimensional time-domain inversion method of bedrock incident waves based on design ground motions is established. Then，the spatially non-uniform free field under the oblique incidence of multi-wave combination is constructed. Finally，the free field deviations constructed based on one-dimensional and two-dimensional inversion methods are analyzed，and the influence mechanisms of different inversion and free field construction methods on the seismic response of the asphalt concrete core dam are revealed. The results show that the free field constructed based on the three-dimensional inversion method of ground motion has spatial inconsistency and can reflect the site ground motion field more comprehensively. Compared with the ground motion field constructed based on three-dimensional inversion，when the incident azimuth angle is 0°~60° and the oblique incident angle is 40°~90°，the feature point free field in the x-direction constructed by the one-dimensional inversion method has a large deviation with the maximum deviation55.8%； When the incident direction is parallel to a certain horizontal coordinate axis，the other horizontal free field deviation of the feature point constructed by the two-dimensional inversion is larger，and the maximum deviation is 100%； When the Poisson's ratio of the medium increases，the deviation of the x-direction free field decreases under the one-dimensional inversion，and the free-field deviation of the x-direction increases under the two-dimensional inversion，and the maximum deviations of the feature points are 46% and 36%，respectively. Compared with the maximum tensile stress of the core wall based on the three-dimensional inversion method of ground motion，the maximum tensile stresses under the two-dimensional and one-dimensional inversion methods are reduced by 83.3% and 20.0%，respectively. Therefore，it is necessary to perform a three-dimensional inversion of bedrock incident waves and construct a spatially non-uniform free field based on the design ground motion in the seismic design of dams.