The effect of process parameters on the filling and contour defects of SLM AlSi12 alloy was studied systematically, and the mechanism of different defects was analyzed, and the controlling method was proposed. The results show that AlSi12 alloy is free from cracks, exhibiting only gas and unfused pores. In the filling area, the pores change from unfused pores to gas pores with the increase of laser power. However, pores changed from gas pores to unfused pores with the increase of the hatch spacing and scanning speed. In the contour area, unfused pores appear when the line energy density is too low(0.10 J/mm), and the contour and filling area can not be well combined. The contour and the filling area are well combined when the line energy density is too high(0.60 J/mm), but the pores appear. The mechanism of unfused pores is: the input energy of powder bed is low, the liquid phase is less and the viscosity is higher, the liquid can not be well spread. The mechanism of gas pores is: the input energy of powder bed is high, the metal is vaporized strongly and the melting pool is stirred violently, and the involved protective gas is difficult to escape under the keyhole mode. By adjusting the process parameters to control the depth and width of melting pool, the good overlap is achieved and bubble escape is guaranteed. Consequently, the filling area and the contour area are densely fabricated, and the density reaches 99.9%. The mechanical properties are excellent：the ultimate tensile strength, yield strength and elongation of AlSi12 fabricated by optimal process parameters are 422.6~446.9 MPa, 280.4~295.4 MPa and 9.8~13.0% respectively, which are significantly superior to those of ZL108 alloy.