Calculated phase diagram reveals important phase details for a new type muti-component magnesium alloy. Isothermal sections at 25°C of Mg-Ca-Sr, , Mg-2.74Al-Ca-Sr, Mg-2.74Al-0.79Zn-Ca-Sr system in the Mg-rich region were calculated using CALPHAD method, Thermo-Calc software and Mg-Al-Zn-Ca-Sr quinary database. The equilibrium phase composition in the Mg-rich region was analyzed. The equilibrium phases and solidification process of Mg-2.74Al-0.79Zn-0.68Ca-0.03Sr quinary alloy was calculated. The calculation results were consistent with the experimental results of SEM-EDX analysis and DSC spectrum. The solidification process of Mg-2.74Al-0.79Zn-0.68Ca-0.03Sr quinary alloy is as follow : α-Mg precipitates at 621°C, secondary phase C15-Al2(Ca, Sr) and Gamma-Mg17Al12 precipitate at around 520°C and 160°C in sequence, A4B-(Al4Ca,Al4Sr) may precipitate at around 460°C.