Abstract: Casting temperature is the main influencing factor on microstructure in the continuous rheo-extrusion process of AZ31 magnesium alloy. The microstructures of AZ31 magnesium alloy at different positions in the roll-shoe die cavity during continuous rheo-extrusion were investigated at different casting temperatures. The results showed that the semi-solid zone becomes smaller when the casting temperature is higher than 750°C, and the time the work roll takes to prune the dendrite in the semi-solid zone is shortened, i.e., the dendrite has not been fully broken up, thus making the high-quality semi-solid alloy unavailable. On the other hand, when the casting temperature is lower than 730°C, the solid-state zone becomes bigger and the semi-solid zone where part of the dendrite has not been fully pruned is merged into the solid-state zone, thus making the dentrite in solid-state zone more difficult to be broken up. As a result, it's impossible to get an ideal semi-solid microstructure. With the solidification zone thus increased, the alloy is deformed more difficultly with increasing working pressure needed for the equipment system and its service life shortened. The best casting temperature is 730~750°C in the continuous rheo-extrusion process of AZ31 alloy.