Adopting vacuum induction melting and step copper mould casting method, non-equilibrium solidified AZ91 magnesium alloy containing SiC particle was fabricated. Then, the microstructure evolution regularity of sub-rapid solidified alloys was revealed considering the effects of heterogeneous particle and cooling rate. In the condition of same inner diameter of copper mould, the addition of SiC particle is beneficial for the refinement of magnesium alloy. After solid solution treatment at 400℃, the β-Mg17Al12 phase at the grain boundary disappears gradually, accompanied by the morphology transition of primary α-Mg from fine granular to polygonal structure. Due to the pinning effect of SiC for grain growth at higher temperature, the initial refined microstructure can be saved after 2h and the whole grain size distribution becomes homogeneous. With decreasing the diameter of copper mould, the cooling rate during solidification is increased and the grain refinement is enhanced as well. As for the specimen with the addition of 2wt% SiC and diameter of 2mm, maximum hardness with the value of 142Hv can be obtained, which is increased by 87% in comparison with the as-cast alloy.