Abstract: To reduce the negative influence of welding residual stress on the welded structure and guarantee the reliability of structure, it is necessary to explore the distribution of residual stress in the welded structure. Modal test method has been gradually applied in the measurement of residual stress due to its fast and non-destructive characteristics. In order to measure welding residual stress by modal test method, the mechanism of modal test method was analyzed by vibration theory, and the relationship between residual stress and natural frequency in thin plate welded structure was obtained. The residual tensile stress in the thin plate welded structure can cause the decrease of natural frequency, while the residual compressive stress can cause the increase of natural frequency. On the basis of theoretical analysis, the hole-drilling method and modal test experiment were combined, and the model parameters for measuring the residual stress of 6 mm 7A52 aluminum alloy(variable polarity plasma arc-metal inert gas, VPPA-MIG) hybrid welded plate using modal test method were estimated through the method of data fitting. The numerical relationship between natural frequency and residual stress was established. According to the established numerical relationship, the longitudinal residual stress distribution of 7A52 aluminum alloy plates under different hybrid welding parameters were measured through modal test experiment. The results illustrate that the distribution characteristics of residual stress measured by modal test method in each zone of the hybrid welded joint of 7A52 aluminum alloy plates are consistent with the results measured by hole-drilling method, and the deviation between the residual stress values measured by the two methods is within the range of 4%. Therefore, the estimated model parameters can guarantee the reliability of the measurement results of the modal test method. It has been achieved fast and non-destructive measurement of the welding residual stress using modal test method.