In order to investigate the characteristics of high-speed trimaran’s resistance and flow field under shallow water conditions, a numerical method was established based on URANS equation and overset grid technology. Numerical simulations were carried out and the influences of water depths on resistance, navigational attitude and flow field of high-speed trimaran were analyzed. The results indicate that the resistance of high-speed trimaran under shallow water is mainly affected by wave-making and depicts different variation characteristics at different speeds. At Fr=0.439, the hull resistance in shallow water increases by 4.64%～23.04% compared to resistance in deep water and reaches the peak at critical water depth. At Fr=0.658, the hull resistance increases by 2.07% at critical water depth. With the depth continuing to decrease, the hull resistance decreases by 3.61%～4.86% compared to resistance in deep water. The change of water depth has little effect on the frictional resistance, while has a great effect on the residual resistance. With the decrease of water depth, the heave of the high-speed trimaran changes from sinking to lifting, the trim increases and the axial nominal wake fraction decreases. Around the critical water depth, there is a significant wave crest in the area between the bow of the main hull and the front of the side hull at Fr=0.439.