Based on Richards-Wolf vectorial diffraction method, we investigate the effect of primary spherical aberration on radiation forces of Rayleigh particles when a radially polarized beam is focused by a high numerical-aperture system. The distributions of radiation forces along axial direction varing with coefficient of spherical aberration are obtained through numerical calculations. The results show that when the primary spherical aberration is changed, influences reflect not only on the magnitude of the radiation forces, but also on the balance position and the valid area. These changes caused by positive and negative primary spherical aberration are not always in the same patterns. The positive spherical aberration makes the gradient forces shift from oposition to the positive direction of z axis while the negative spherical aberration makes the opposite shift. Under different numerical apertures, the ratio of aperture and waist length and particle radius, the influcences of primary spherical aberration on optical trapping force will change.