Mobile offshore double-causeway pier system, a type of seashore unloading equipment, consists of two groups of multiple connected semi-submersible modules. This structure has wide application because most of the middle or mini type of vessels and ships can be moored to it. Based on the analysis of computational methods of multi-body motion response, a hydrodynamic model is set up and the three-dimensional potential theory in finite depth is adopted to calculate the three-dimensional motion response of this system. The double P-M spectrum is used to analyze the motion response in irregular waves. Different wave directions are specially taken into consideration, due to their various effects to the motion response. Furthermore, the calculated result is compared with that of the experiment, and it is proved that sway, heave, pitch and yaw motion are greatly constrained by mooring system. The comparison also indicates that the model can forecast the motion performance of the target, and that the calculated result can also be used as reference in connector and mooring system design.