In forward modeling methods of surface seismic, only upgoing reflection waves were considered, and the information such as transmission wave, interlayer direct wave and downgoing wave were not involved. In this paper, a kinematic forward modeling method for crosswell seismic is introduced. First, a model building method based on block units is used to build a crosswell seismic geological model. Then, the trial-ray method is used for ray tracing of the geological model. Finally, crosswell synthetic data are obtained. The results show that this forward modeling method based on block units can simulate complex geological structures with faults, pinchouts and so on.