In the process of manufacturing and service of the thin metal plate structure, it is usually accompanied by the residual stress and external stress. Excessive stress will cause the thin plate to deform and seriously affect the normal operation of the overall structure. The evaluation of the stress state is an important index for judging whether the engineering component is safe and reliable, and the ultrasonic method is an effective non-destructive measurement method for stress. However, the current ultrasonic stress measurement technology has low sensitivity and there is little study on it. A thin plate stress measurement technology based on ultrasonic high-order Lamb waves is proposed in this paper. First, based on the finite element characteristic frequency method with Bloch-Floquet boundary and domain constraints, the acoustoelastic effect of each high-order mode of the Lamb wave is studied, and the mode sensitive to stress is selected. Then, an ultrasonic Lamb wave measurement system is built. The A1 mode at a specific frequency is excited and the stress calibration is carried out in a uniform thin plate. Finally, the stress detection was carried out in the non-uniform thin plate, the stress at different positions in the thin plate is successfully detected, and the maximum stress error was 15 MPa. The results show that this work verifies the proposed thin plate stress measurement technology based on ultrasonic high-order Lamb wave from the theoretical and experimental aspects, and the stress in metal plate can be measured accurately.