Mg-10Y-2.45Znand Mg-10Y-2.45Zn-1Ca-0.37Al alloys were fabricated by magnetic levitation induction smelting coupled with copper-mould suction casting technique. The compound effect of high content Ca and small amount of Al alloying element addition on the formation of LPSO phase in as-cast Mg-10Y-2.45Zn alloy and the microstructure evolution and mechanical behavior under high temperature and high stress were investigated. The results showed that high content of Ca and a small amount of Al were involved in the formation of massive 18R LPSO and a large number of 18R LPSO phase delamination interface emerged under the as-cast condition. At high temperature, unlike the Mg-10Y-2.45Zn alloy, a large number of 18R LPSO phases dissolved and grew at the grain boundaries of Mg-10Y-2.45Zn-1Ca-0.37Al, the size of α-Mg grains coarsen. At high temperature and high plastic deformation, massive 18R LPSO kinked, delamination or crushed, dynamic recrystallization occured at the vicinity of the 18R LPSO phase boundary and a large number of fine rod-like 14H LPSO phases precipitatied in α-Mg matrix conspire to strengthen the alloy.