Abstract: The propagation, transmission and reflection characteristics of seismic waves in overburden sites are extremely complex. The reasonable construction and expression of the free field of bedrock-overburden sites is the key to generating nonuniform ground motion input. The expressions between two directional ground motions at any point in an overburden site space and the incident SV and P waves are established. The time history and incident angle of the incident SV and P waves can be obtained from the measured ground motions in the horizontal and vertical directions of the ground surface. By considering the multiple reflections at the interface and the amplitude attenuation caused by soil damping, the free field of the bedrock-overburden site space is constructed. The results of the comparison with the measured array records at La Cienega site show that the simulated values of the two directional ground motions at different depths are consistent with the measured records. The time histories of the horizontal and vertical ground motions at any point in the free field are inconsistent. The spatial free field construction method proposed in this paper provides a basis for non-uniform ground motion input of overburden-dam systems.