In this article, the physical equations of spherical objects moving in the viscous liquid have been formulated. These equations are then solved by the method of fourth-order Runge-Kutta to obtain numerical simulations and the law of the motions of soybeans, rice and wheat in the viscous liquid and analyze the factors affecting the motions of seeds underwater. The simulation results show that, for the homogeneous seeds, the velocities are proportional to the radiuses, the longer the radiuses are, the higher the ending velocities. However, for different kinds of seeds, the velocity variables are proportional to the seed densities, the higher the densities are, the higher the accelerations. Therefore, the above laws would be helpful to further study the motions of crops underwater and select and cultivate seeds.