Using laser selective melting forming (SLM), casting, extrusion way prepared WE43 magnesium alloy specimens, through the Vickers hardness tester, density tester, optical microscope, scanning electron microscope, as well as tensile tester and other equipment to analyze the macro and microstructure and mechanical properties of the different preparation way forming WE43 magnesium alloy change rule. The models based on exponential function were designed to uniformly fit the stress–strain curves of WE43 with different forming methods, which laid a foundation for the future composite manufacturing of complex parts by additive, subtractive and equal material processes of WE43. The results show that SLM forming WE43 magnesium alloy has the highest strength, the tensile strength reaches 313 MPa, which is 183% of the casting state; extruded state of WE43 magnesium alloy has the best plasticity, the elongation rate reaches 10.2%, which is 232% of the casting state; the density of magnesium alloy of the SLM state is only 1.731 g/cm³, which is only 85.7% of the extruded state, and 95.2% of the casting state. The SLM state and the extruded state are ductile fracture, while the casting state is brittle fracture. SLM-formed WE43 has obvious anisotropy, the casting state and extruded state is not obvious. In the presence of a large number of hole-shaped defects around 20 μm, SLM-formed magnesium alloys still have the highest strength, mainly due to the average grain size of SLM-formed WE43 magnesium alloys of only 2.6 μm, the large amount of rare-earth phase precipitation present in the matrix, and the nanoscale sub-stable phases. It can be seen that the mechanical properties of the material can be significantly improved after welding and closing the internal hole-shaped defects in the SLM state magnesium alloy by further post-treatment methods.