Abstract: The change of microstructure and mechanical properties of AlSi10Mg alloy fabricated by selective laser melting（SLM） before and after heat treatment was studied. The phase composition and microstructure of AlSi10Mg alloy formed by SLM under different annealing process were studied by XRD, SEM and other means, and the microhardness and tensile strength of the formed alloy after different annealing processes were measured. The results show that the phase of the AlSi10Mg alloy formed by the SLM is mainly α-Al solid solution and co-crystal（α-Al+simple Si）. The grain shape is mainly columnar crystal which grows from the edge of molten pool to the center. The Vickers hardness of the alloy is 125 HV0.5, the tensile strength is 465 MPa, and the elongation at break is 7.5%. After annealing, the cocrystal is dissolved into α-Al matrix in different degrees, and its tensile strength and hardness decrease with the increase of annealing degree. The tensile strength of the alloy decreases to 221 MPa, and the plasticity increases with the increase of annealing degree, and the elongation increases to 23.5%.