The influencing factors of surface alloying layer by evaporative pattern casting technology were investigated. A certain thickness alloying layer was formed on the surface of Mg-alloy matrix when the pouring temperature was 780 ℃ with different vacuum degree and alloying powder size. The surface layer microstructure, micro area composition of the new phase formed on the matrix and the composition characteristics on the surface layer were examined by SEM and element scanning. The results show that the content of aluminum increases greatly on the surface layer. The micro-hardness of alloyed layer has a more obvious increase compared with that of the matrix. The size of alloying element and the vacuum degree are the key factors influencing the alloying layer, with the increase of element powder size from 0.074 to 0.15 mm and vacuum degree from 0.04 to 0.06 MPa, the surface alloying effect becomes better.