The phase evolution in (88%-91%)Mg-8%Sn-l%Zn-X (X=A1, Mn and/or Ce) system was analyzed via CALPHAD method and simulations were used in precise selection of the chemical composition. The influence of the addition of different alloying elements such as Al, Mn and Ce on the microstructure and microhardness of Mg-8%Sn-l%Zn-based alloys was investigated. Combined addition of Al and Mn shows features distinct from separate addition of Al or Mn. Additions of 1%A1 and l%Mn to base alloy result in the formation of massive Al-Mn phase in a-Mg matrix grains. Addition of Ce element can refine the second eutectic precipitates and form intermetallic compounds with Sn. Fine rod-like Sn-Ce phase presents mainly on the grain boundaries and plays a role in inhibiting grain growth. The effects of alloying elements on Vickers microhardness and indentation size effect of base alloy were examined.