The effect of the different stress ratios (R) and annealing treatment on the fatigue properties of the transparent polycarbonate (PC) sheet and the mechanism behind were studied, the fatigue crack propagation (FCP) process and mechanism were analyzed. The results show that after annealing, the residual stress of the PC samples decreases obviously and the fatigue properties are greatly improved. This is because the machining process results in tensile stress in the PC samples, eliminating the tensile stress can improve the fatigue property of PC effectively; under the positive stress ratio (tensile-tensile fatigue), as the stress amplitude increases, the fatigue properties of PC decreases obviously, however, the negative stress ratio (tensile-compression fatigue) causes complex influence on the fatigue properties of PC. The fatigue fracture morphology suggests that distinct crack source, stable crack extension zone and instability zone exist obviously in fatigue fracture of PC. The mechanism of the fatigue crack propagation of PC indicates that crazes are initiated at first under the cycling stress, when the stress is higher than the strength of the fibrils in crazes or the fibrils are broken under the cycling stress, crazes develop into cracks.