A calculation method of fatigue life for main shaft bearing of direct drive wind turbine was proposed. The statics models of the bearing were established and a set of equilibrium equations were obtained. By solving the equilibrium equations, the displacements of the inner ring were obtained, and the rolling element loads were calculated further. Based on the load distribution of the rollers, the fatigue life of the bearing was calculated and the influences of the clearances on it were researched. Results show that the clearances of the bearing can influence its service life distinctly. Zero axial clearance is recommended for the bearing.