The invention discloses a magnesium base composite material with a high-entropy alloy as a reinforcing base and a preparation method of the magnesium base composite material. The composite material uses the high-entropy alloy as the reinforcing base and a magnesium alloy as a matrix; and the specific preparation method comprises the following steps: (1) metal powder is respectively weighed and mixed according to the ratio of the high-entropy alloy; (2) the mixed metal powder is put on a ball mill for ball milling to prepare high-entropy alloy powder by a mechanical alloying mode; (3) the high-entropy alloy powder is put in a drying box for drying; (4) the dried high-entropy alloy powder is screened out alloyed alloy powder, and then, is put in a vacuum environment for preservation; and (5) the high-entropy alloy powder and a high-purity magnesium alloy cast ingot are smelted in a well furnace under argon atmosphere to prepare the magnesium base composite material with the high-entropy alloy as the reinforcing base. The magnesium base composite material is prepared by the casting mode; the high-entropy alloy powder and the magnesium alloy matrix are better in wettability; the uniform dispersion of the high-entropy alloy is facilitated; and the interface bonding strength is higher.