A new technique for determining the plastic strain ratio of sheet metal is proposed. After off-line calibration of the two cameras, digital images of the process of sheet metal surface deformation were temporally taken by the cameras so that two sets of time-synchronization image serial were obtained, the 3D shapes of sheet metal surfaces at different deformation time were reconstructed by using stereo vision and digital image correlation methods consequently. Local major and minor strains were then evaluated in terms of finite strain theory. By exploiting the approximate equality of local strains at the same moment during uniform plastic deformation, the major and minor strain were converted into the strains in length and width directions, from which r value can finally be determined. Under the experimental conditions prescribed in GB/T 5027-2007, r values of AA6016T4PD, 2A12-T4 2 and 2A12-O were determined, and the results demonstrated the proposed technique was accurate and rapid.