Abstract: Gleeble 3800 thermal simulation test machine was employed to simulate the welding thermal cycling process of AZ31B magnesium alloy at different peak temperatures. The relationship between residual stresses and peak temperatures was obtained, and a mechanical melting point testing method for AZ31B magnesium alloy was established. The thermal-electric couple numerical simulation method was used to simulate the thermal cycle process of the thermal simulation test, and the thermal cycling curves were obtained at different positions from the center of the thermal simulation sample as a heat source model for simulation. The thermo-elasto-plastic finite element method was used to calculate the stress evolution during the thermal simulation test process, and different mechanical melting point values were set to analyze the influence of mechanical melting point on residual stress, which can verify the reliability of the mechanical melting point test results of AZ31B magnesium alloy. The results show that the mechanical melting point starting temperature of AZ31B magnesium alloy is 459 ℃, and the ending temperature is 526 ℃. In numerical simulation, the higher the mechanical melting point, the greater the calculated residual stress. The calculated stress using the starting temperature and ending temperature as the mechanical melting point of AZ31B magnesium alloy is consistent with the measured values.