Objective：To prepare a novel magnesium alloy stent using 3D printing and investigate its microstructure and in vitro degradation performance. Methods：Magics software was applied to design a cylinder 3D model，on the basis of which，3D-printed magnesium alloy（3D-AZ91）specimens were prepared with AZ91 magnesium alloy powder via an additive manufacturing technology. Then，metallographic structures of 3D-AZ91 and casting magnesium alloy（AZ31）specimens were observed. Surface elements and crystal phase structure were analyzed by EDX and XRD，respectively. Vickers hardness values were tested. After that，the polylactic acid（PLA）was coated on the surface of 3D-AZ91 to make the composite material specimen（PLA-3D-AZ91）. The degradation experiment was performed，and the in vitro degradation properties of three different specimens were measured by observing hydrogen evolution and calculating weight loss rate. The surface morphology of 3D-AZ91 and AZ31 specimens after degradation was evaluated by the scanning electron microscopy. The elemental compositions of the surface degradation products were examined by EDX. Results：Metallographic observation showed more obvious grain refinement in 3D-AZ91 than in AZ31. Mg was main surface element and α-Mg matrix was main crystal phase for both of them. The Vickers hardness of 3D-AZ91 was significantly higher than that of AZ31. The degradation rates were as follows：3D-AZ91＞AZ31＞PLA-3D-AZ91. The surfaces of 3D-AZ91 and AZ31 were covered with irregular agglomerate degradation products，which contained magnesium，calcium，phosphorus and other elements. Conclusion：Compare with the traditional casting method，3D-printed magnesium alloy possesses better mechanical property. What’s more，its degradation rate can be effectively improved by the PLA surface coating.