Aiming at the abnormal situation that the water tank is not full of ballast water, the longitudinal static recovery moment model and calculation method of the single-point mooring soft yoke are established. When the water tank is not full of ballast water, the water will flow in the water tank along with the longitudinal rotation of the soft yoke, causing the mass center of the structure to change. It is found that there is a big difference in the variation curve of the longitudinal static recovery moment of the soft yoke with the rotation angle between this situation and the situation of fixed mass center under the same mass. The smaller the water carrying capacity, the greater the difference between the two situations. With the gradual increase of the carrying water, the difference becomes smaller and smaller, and eventually tends to be the same.