The relationship between the nonlinear effect of Lamb wave and temperature and stress of thermostat metal strip is studied, and the nonlinear ultrasonic testing system is established, and the stress value is determined by X-ray diffraction method. Heat the thermostat metal to 100 ℃, 200 ℃ and 310 ℃ respectively for one hour and then air cool it. Compare the influence of different temperatures on the nonlinear coefficient, and analyze the relationship between the nonlinear coefficient and the stress value of the thermostat metal under normal conditions and after stress reduction by heat treatment. The experimental results show that the nonlinear coefficient of the thermostat metal under normal temperature and after heating at 100 ℃ and 200 ℃ is basically unchanged, but after the stress reduction by heat treatment at 310 ℃. The nonlinear coefficient is significantly reduced. It is verified that the second-order and third-order nonlinear coefficients of Lamb wave in thermostat metal are sensitive to the stress value and increase with the increase of stress, which can be used to characterize the residual stress state. The fitting function can be obtained according to the relationship between the stress value and the nonlinear coefficient, and the feasibility of calculating the stress through the fitting function is verified, which provides a reference method for using the nonlinear coefficient to evaluate the stress.