Negative side wall springback and wall thickness of AZ31B magnesium alloy square conical parts were studied. Orthogonal test with four factors and three levels was designed for the negative springback of the side wall. Firstly, the influence of forming angle, forming temperature, pressure amount and forming height on the negative springback of the side wall of the formed part was studied through the machine tool processing experiment, which was verified by numerical simulation. The results show that the influence degree of negative springback on the side wall of square conical parts from high to low is forming angle, pressure amount, forming temperature and forming height. Both simulation results and machine tool experimental results show that the negative springback section is an important part that affects the springback of forming parts. The simulation results also show that different support modes have important impact on the wall thickness distribution of forming parts. The side wall thickness under the extension mode of the lower pressure plate is more uniform than that under the nonextension mode of the lower pressure plate, and the wall thickness difference is smaller. The springback value of the negative springback section can be effectively reduced by the extension mode of the lower pressure plate.