In this paper, the features of the complex wavefield in project sites are revealed from the physical perspective, and the random wavefield method is introduced to establish the four-dimensional wavefield information consistent with the site spectrum. The influence of the complex wavefield effect on the seismic response of the hardfill dam is exposed through the vertical incidence model and the complex wavefield model. The results show that the wavefield features at the hardfill dam site contain the wavefield information, the propagation of the wavefield, and the radiation-damping effect. The results from the two input models reveal that the isolines of the hardfill dam on peak horizontal acceleration are nearly horizontal, and the ones of peak vertical acceleration are mountain-distributed. The tensile stress appears at the dam heel area, and large compressive stress at the dam toe area. The complex wavefield effect triggers increases in the mean value and range of peaks of the hardfill dam. For the peak σ1 at the dam heel, the mean value is increased by 37.5%, and the range of peaks by 70.2%. The probability density curve is narrow-high type from the vertical incidence model and wide-low type from the complex wavefield model. In the aspect of a single case, the complex wavefield effect may increase or decrease the peak response. In calculation cases, the change-levels of peak σ1 at the dam heel are from -6.3% to 81.9%, of which the increase cases account for 92%.