To evaluate the accuracy reliability of machine tools, an evaluation method of accuracy reliability considering the machining error out of tolerance is proposed, and an accurate mathematical description is given to realize the quantitative evaluation of machining accuracy under different machining loads. On this basis, the basic connotation and mathematical model of accuracy retention are analyzed, and the influence of different machining loads on the accuracy retention is studied. Finally, experiments are carried out to detect the variations of machining errors of machine tools under different loads and the quantitative evaluations of accuracy reliability and retention of machine tools are completed, which verify the validity of the proposed methods.