The invention relates to a method for refining a Mg-Al-based alloy by using nano zinc oxide, which comprises the following steps: (1), preparing aqueous solution of ZnSO4 and solution of NaOH; (2), adding aqueous solution of ZnSO4 dropwise into solution of sodium hydroxide; (3), placing mixed solution in an ultrasonic cleaner for vibrating and stirring for 2 to 3h; (4) centrifuging at a speed of 5,000 to 6,000rpm, cleaning a solid phase after centrifugation, drying and removing water to obtain a nano zinc oxide crystal particle refiner; and (5) placing the crystal particle refiner in a Mg-Al-based alloy melt when the temperature of the Mg-Al-based alloy melt is between 690 and 720 DEG C, stirring uniformly and standing the mixture. The nano zinc oxide is added into the Mg-Al-based alloy melt, the nano zinc oxide refines crystal particles of the Mg alloy through heterogeneous nucleation. The method can easily control the refining effect by adjusting the additive amount, and achieve good refining effect by adding a small quantity of nano zinc oxide.