To realize the in-situ stress measurement of large composite plates, an air-coupled ultrasonic stress measurement method for composite plates based on Lamb waves is proposed. At present, due to the severe impedance mismatch of the air-coupled ultrasonic transducer, energy loss is severe, and the air-coupled ultrasonic signal is relatively weak, making it difficult to accurately extract acoustic features. Meanwhile, the application of the air-coupled ultrasonic Lamb wave stress measurement method in composite materials is still in the exploration stage of theory and experiment. The Lamb wave acoustic elastic effect of carbon fiber composite materials is explored through experimental methods. According to composite plates ’ the phase velocity and the optimal incident dispersion characteristics analysis, the center frequency of the air-coupled ultrasonic transducers, and the excitation method of the relatively pure A0 mode Lamb wave is determined. The accuracy of sound time extraction is ensured by a good signal-to-noise ratio of the spatially coupled ultrasonic Lamb wave. To evaluate the effectiveness of the proposed method, seven different tensile specimens are obtained along the fiber directions of 0°, 15°, 30°, 45°, 60°, 75°, and 90° for stress measurement. The experimental results show that the measurement error is less than ±8. 1 MPa in the range of 0~ 100 MPa, and the measurement repeatability is 7. 5 MPa. This method has significant advantages in measurement accuracy and repeatability and could provide an advanced and feasible technique for in-situ stress measurement of large composite plates.