This research explored the corrosion effect of NaCl solution on the surface quality of AZ31 magnesium alloy. Cubic-shaped specimens were immersed in standard 3.5% NaCl solution for 1, 2, 3, and 4 hours. The size of corrosion pits, fatigue crack propagation area, and surface topology were characterized. Fatigue tests were carried out on pre-corroded AZ31 specimens under different stress amplitudes. The correlation between the corrosion time and the virtual crack size was obtained. The virtual crack size is a new parameter that can relate the corrosion to the fatigue life. A new model was proposed to map the surface topology on the virtual crack size of pre-corroded AZ31 magnesium alloy. The fatigue life of 1-4-hour pre-corroded AZ31 specimens could be predicted accurately by this new model. The Levenberg-Marquardt and curve-fitting methods were used to obtain the constant parameters. The relative error between the crack size corresponding to the immersion time and the virtual crack size calculated by the proposed model was less than 1%.