Abstract: The present fatigue life evaluation approach for aluminum alloy containing defects is based on depth extension of crack which is hidden inside and hard to be directly obtained. In order to find the relation between depth extension and surface cracks length, fatigue experiment was carried out on aluminum alloy 7A09 with prior corrosion damage, and propagation rules of corrosion pit cracks along depth direction was found by analyzing the fatigue fracture by using scanning electron microscope and energy disperse microscope. Results indicate that the fatigue fratography exhibits brittle fracture morphology, and the accumulation of the second phase particles will inversely enhance embrittlement by reducing plasticity. For fracture dominated by solo crack, the depth extension and the surface length of crack displays a linear relationship which provide a way to evaluate depth extension of a crack based on the detectable surface length, which is valuable to residual life prediction of structure. For multiple fatigue, there is uncertain relation between extension length and depth of crack.