The influence of the side wall effect on the ship motion should be taken into account to predict the motion of the ship in the restricted waters, such as a narrow channel or a canal. The side wall effect has been considered by introducing the green function of channel free surface. The green function of the side wall with free surface is then approached by Chebyshev polynomial to perform the efficient and high accurate calculations. The results calculated in the current study are compared with which is calculated by the commercial software. The artificial numerical damping is adopted for the free surface in order to address the numerical resonance phenomenon of the free surface during the numerical calculation, ensuring that the motion of the floating body and the calculated elevation of the free surface are more accordant with the actual conditions.