A two-step superplastic deformation (SPD) approach was employed for the SPD of Mg-9Li-1Al (LA91) alloy. In the first SPD step, the constant velocity (Constv) or constant strain rate (CSR) SPD was adopted with the outcome elongation ranging from 50 to 250%. The maximum strain rate sensitivity (Maxm) SPD, in which the maximum m value is always maintained, was then employed in the second SPD step. The results show that the two-step SPD can greatly enhance the superplasticity of LA91 alloy. The largest elongation of 621.1% was obtained by CSR-Maxm SPD with the pre-elongation of 250%, which is even better than the results of single-step Maxm SPD by using grain-refined materials. In addition, the mechanisms of the two-step SPD are discussed and an in-depth understanding of the SPD of LA91 alloy is obtained. This research thus provides a feasible method for efficiently enhancing the plasticity of Mg-Li based alloys for making complex parts and structures with complicated local and multi-scaled features.