Effects of Ca and Si compound alloying on as-cast microstructure of AZ31 magnesium alloy were analyzed by optical microscope （OM）,SEM （scanning electron microscope）,XRD （X-ray diffraction） and EDS （energy dispersive spectrometer）. It is found that Ca element exists in the AZ31 magnesium in form of solid solution phase,and Si element is irrelative with the distribution of Ca in the alloy. Mg2Si phase is created by Si reacting with Mg,meanwhile,Mg17Al12 phase distributed along boundary is gradually converted into particle and acicular phase dispersedly distributes around boundary or in the grain. With increasing in Si addition,partial particles become coarse,and Chinese script Mg2Si phase appears in the matrix alloy. The desirable as-cast microstructure of the alloy can be observed with 0.3% Ca and 0.4% Si compound addition.