The invention discloses a preparation method of fine-grain magnesium alloy doped with nano-particles. The method comprises the following steps: firstly doping the nano-particles in a magnesium alloy melt, applying ultrasonic vibration on the magnesium alloy melt doped with the nano-particles, and condensing under the effect of secondary ultrasonic vibration to obtain magnesium alloy doped with the nano-particles; finally putting the magnesium alloy doped with the nano-particles in a processing mould for heat preservation for a certain period of time, carrying out multi-step temperature change compression at the compression temperature of 300-400 DEG C so as to obtain the fine-grain magnesium alloy doped with nano-particles through three to six steps of compression. The fine-grain magnesium alloy doped with nano-particles is successfully prepared by firstly adopting a two-step compound technology of ultrasonic vibration and multi-step temperature compression, so that clustering of the nano-particles caused by pushing of a solid liquid interface to the nano-particles during the solidification process is effectively solved, and the grain of the magnesium alloy doped with nano-particles is obviously reduced through the multi-step temperature change compression. The preparation method is mainly used for preparing the fine-grain magnesium alloy doped with nano-particles.