Reasonable ground motion input is very important for studies of earthquake hazard analysis and the seismic design of engineering structures in high seismic intensity area. Due to the fact that the ground motion characteristics are strongly affected by the slip distribution of earthquake rupture, the source parameters and the relationship between the moment magnitude are established from the physical mechanism of earthquake source. We establish the multi-parameter correlation random field source model about the source, the rise time, and the scaling of rupture to show the random characteristics of seismic source better. Choose three commonly used correlation functions such as the Gaussian, exponential and Von Karman correlation function, to control the spatial statistics of slip distribution and define the covariance matrix. On this basis, this study attempts to use the Green’s function to describe the uncertainties in earthquake source and the path of seismic waves. Then, it establishes a bedrock seismic motion model which can reflect the effect of earthquake source uncertainties, and take Cascadia subduction zone as an example, uses the improved spatial random field model to simulate the stochastic slip of faults and obtain the random earthquake ground motion. The final results show the model is a good method for strong ground motion simulation.