The invention provides high-strength high-temperature-creep-resistant die-cast magnesium alloy and a preparation method thereof and belongs to the technical field of low-density alloy and machining. The die-cast magnesium alloy comprises the following components in mass percentage: 7.0wt% to 11.0% of Al, 0.5wt% to 2.5wt% of Zn, 0.2wt% to 0.4wt% of Mn, 0.05wt% to 2.0wt% of enriched Y, 0.2wt% to 3.5wt% of Yb, 0.2wt% to 6.0wt% of Sm, 0.1wt% to 2.5wt% of Sr and the balance Mg and inevitable impurity elements. The invention also provides the preparation method of the high-strength high-temperature-creep-resistant die-cast magnesium alloy. The magnesium alloy prepared by the method provided by the invention is high in strength and good in high-temperature creep resistance. Test results show that under a condition that the applied stress is 50MPa and the test temperature is 160 DEG C, the 100h steady-state creep rate of the magnesium alloy is 1.05X10<-9>s<-1>, the 100h creep strain is 0.14%, and the room-temperature tensile strength, yield strength and elongation rate are separately 293MPa, 202MPa and 8.6%.