The linear relationship between the standard deviations and means of lgNf in different strain levels is discovered by the statistical analysis of the HCF and LCF fatigue test data of AL 7050-T7451, and a strain-based probabilistic model of fatigue life is presented, which is suitable for HCF and LCF. Furthermore, the probabilistic model with confidence of 0.95 is established based on the interval estimation of the standard deviations and means of lgNf. The fatigue reliability of a mechanical hand on space shuttle is analyzed by this model, and the distribution of fatigue life and reliable life with reliability of 0.9987 are obtained.