The invention discloses a grain refiner for magnesium-aluminum (Mg-Al) magnesium alloy and a preparation method thereof. The grain refiner consists of a calcium magnesium stannum (CaMgSn) intermediate compound and magnesium, wherein the weight content of impurities is less than or equal to 0.3 percent; and the CaMgSn intermediate compound with small granularity is used as a heterogeneous nucleation core of a substrate in solidification, the formed crystal particle is fine, the addition quantity is stable, and the crystal particle is high efficient to refine and stable. The refiner is prepared in a vacuum induction furnace according to a reasonable formula, the burning of elements can be effectively avoided, the controllability of components is strong, CaMgSn can be dispersed and distributed in the substrate through squeezing, the size is fine, and the refining capacity is further improved. In addition, The AZ31 aluminum-magnesium alloy crystal particle is refined according to the formula and process disclosed by the invention, the size of the cast crystal particle of the magnesium alloy is 70 micrometers, and the crystal particle refining effect is remarkable compared with the size 170 micrometers of the crystal particle of the magnesium alloy without the crystal particle refiner under the same state.