The boundary-layer feature and the forces on the particle are analyzed in detail, and the motion parameters of the particle in the gas-solid rotary flow are divided into two parts according to the meridian and cross section. The Lagrange method is then applied, the 3-D mathematical model of particle motion in the gas-solid rotary flow is presented, and the Gear integral method is applied to simulate the motion characteristics of the particles. The results show that the centrifugal force and Saffman lift force play important roles in the process of the particle being separated from the gas-solid rotary flow in the rotary boundary layer. The velocity gradient of radial direction is the biggest, and that of tangent direction is the smallest. For a higher density ratio of gas to solid, the deposition performance of the particle depends not only on the inlet flow velocity but also on the range of the particle diameter. Reasonable velocity gradient matching of the three directions in the gas-solid rotary flow is useful to improve the separation efficiency of the rotary separators.