The invention discloses a semi-continuous casting process of large-diameter moderate-strength heat-resisting magnesium alloy cast ingots, wherein the alloy at which the semi-continuous casting process targets comprises the following ingredients (wt%): 7.5-9.0% of Al, 0.02-0.80% of Ag, 0.35-0.55% of Zn, 0.05-0.20% of Mn, 0.01-0.10% of Re and 0.001-0.020% of Ca, and the balance of Mg and inevitable impurity elements. The process comprises the following steps: smelting raw materials at a temperature ranging from 620 to 680 DEG C, introducing gas and stirring, increasing the temperature to the range from 720 to 740 DEG C and preserving heat for 10-20 minutes, standing at a temperature ranging from 680 to 700 DEG C, standing for 1-3 hours, and casting at a low temperature and the like. The magnesium alloy cast ingots having the diameter phi ranging from 330 to 630mm and the length greater than or equal to 1000mm can be cast successfully by adjusting the process parameters of all the steps. With the large-diameter and high-quality cast ingots, good-quality blanks can be provided for the formation of large-dimension structural members, and therefore, the continuous casting process of the large-diameter moderate-strength heat-resisting magnesium alloy cast ingots plays an important role in boosting wide application of the magnesium alloy.