In order to predict the crack defects in sheet metal bending, an Lemaitre criterion was improved to effectively predict the forming limit in bending while considering the stress triaxiality , the maximum principal stress ratio , and the influence of plastic strain on the damage. Taking 7075-T6 aluminum alloy as the research object, the crack forming in sheet bending was simulated to obtain the criterion parameter so as to determine the cracking threshold. Three point bending test and metallographic test were carried out on 7075-T6 aluminum alloy sheet with 6 mm in thickness. The accuracy of the criterion for crack prediction was verified by comparing the press measure of experimental, simulation and theoretical values. The results show that the improved Lemaitre fracture criterion is 9.7 mm, which is consistent with the results of simulation and experiment. Therefore the improved Lemaitre fracture criterion has a certain accuracy in predicting the crack of bending forming.