Most previous studies focused on the overall force on a cylinder, and the detailed mechanical characteristics for different parts of a cylinder were less studied. A 3-D numerical wave flume was used to investigate the internal solitary wave forces acting on cylinders by large-eddy simulation(LES). Through analyzing the hydrodynamic characteristics and the pressure distribution around cylinders, the detailed vertical distribution of the pressure drag force and viscous force was obtained. Based on multi-linear regression analysis, the comprehensive impact parameters were constructed to establish an empirical function of the force coefficients for the cylinders. The results show that the pressure drag on a square cylinder is 1. 5 times more than that on a circular one in the upper layer, while the pressure drag on a square cylinder is 3. 5 times more than that on its circular counterpart in the lower layer. The viscous force is 1 or 2 order of magnitudes smaller than the pressure drag, which means that the viscous force can be ignored. The comprehensive impact parameters show a positive correlation with the relative wave height, and show a negative correlation with the upper and lower water depth ratio.