Residual stress may be generated during the deformation process; cold and hot treatments on magnesium alloy, causing deformation; cracking; and other effects. Reducing the residual stress of magnesium alloys is of great significance for its size stability and quality. In this paper, the residual stress in the AZ31B plate was compared with different uphill quenching processes: no uphill quenching (NUQ), liquid nitrogen–boiling water (100 °C) (LNB), liquid nitrogen–hot air (160 °C) (LNHA) and liquid nitrogen–water (25 °C) (LNR). Residual stresses with and without treatment were measured by X-ray diffraction. The effect of uphill quenching on hardness was discussed. The microstructure and diffraction pattern of the samples treated with different uphill quenching processes was investigated by EBSD and XRD. The results showed that the microstructure of magnesium alloy rolling plate was refined by the uphill quenching treatment, which can reduce the residual stress without decreasing the mechanical properties. The largest residual stress reduction rate was obtained by the liquid nitrogen–boiling water process. This treatment process can not only reduce the residual stress of the magnesium alloy rolling plates by 56% but also increase the hardness by 29%.