Bolted sphere joints in spatial grid structure are always susceptible to fatigue failure. Based on linear elastic fracture mechanics，the stress concentration factor of bolt thread was used to estimate the fatigue ultimate strength of bolted sphere joints. Based on S-N curve method, a simplified formula to estimate fatigue life of bolted sphere joints was derived by considering the effects of stress amplitude and stress ratio on fatigue life in combination with equilife curve. Compared with the existing experimental results, the predicted formulas can satisfy the engineering design requirements. On this basis, the effects of initial crack length on the fatigue lifetime of high strength bolts were studied and the influence rule was given.