A high-strength high-modulus 
magnesium alloy comprises 3-20% of alloying element and the balance of 
magnesium substrate, wherein the alloying element comprises heavy 
rare earth, 
silicon and / or 
germanium, and 
tin and / or 
antimony and / or 
zinc; Si and Ge as well as Sn, Sb and Zn can be simultaneously added and can also be added individually; and the heavy 
rare earth is selected from at least one of 
gadolinium, 
dysprosium, 
terbium, 
holmium erbium, 
thulium, 
ytterbium and 
lutetium. The preparation method comprises the steps of: heating and melting pure 
magnesium in protective 
atmosphere, adding pure 
silicon to the pure magnesium melt, stirring, dissolving, heating-up to 20-30 DEG C, sequentially adding an interalloy of the other components, stirring, controlling the interalloy to be completely molten until the 
casting time is less than or equal to 4min, and 
casting to obtain a cast 
ingot. The high-strength high-modulus 
magnesium alloy is reasonable in component proportioning, and easy to process and manufacture; and the prepared high-strength high-modulus 
magnesium alloy has high room-temperature strength and elasticity modulus and better 
plasticity, and comprehensive performances apparently higher than those of the existing magnesium-
rare earth, and is suitable for industrial production.