For a SWATH equipped with stable fins, numerical calculations were conducted at a speed of 0.5 with the Froude number. The results showed that the SWATH’s heave and pitch motion stability was guaranteed after the installation of stable fins, proving the advantage of stable fins for heave and pitch motion stability. By studying the influence of the longitudinal installation position, area, and combination method of the stable fins on the heave and pitch motion stability, it was analyzed that the tail fin area of the ship needs to reach 6% of the submerged body projection area to ensure heave and pitch motion stability. Taking 10% to 20% of the tail fin area for the head fin can achieve good heave and pitch motion performance and have a significant beneficial effect on the wave resistance performance.