In this work, an online static magnetic field system for the SLM process is designed. An optical microscope and a scanning electron microscope are employed to examine the microstructures of the specimens, and a material testing machine is used for the mechanical property tests. Our findings show that applying a magnetic field can significantly improve the relative density of the formed sample. As the magnetic field strength increases from 0 to 0.3 T, the grain orientation changes from the strong (001) orientation to (001), (101), and (111) uniform orientations, while the texture index decreases from 6.843 to 5.718. Thus, to a certain extent, the anisotropy of grain orientation of GH3536 alloy is weakened. The results suggest that the change in grain orientation is mainly due to the generation of thermoelectric forces acting on the dendrites. Besides, the applied magnetic field makes the tensile strength and elongation of the sample along the scanning and deposition directions tend to increase, especially along the scanning direction. The tensile strength along the scanning direction under the 0.1 T and 0.3 T magnetic fields increases by 20 MPa and 70 MPa, respectively, and the elongation increases by 0.8% and 2.9%, respectively. At 0.1 T and 0.3 T, the differences in tensile strength along the scanning and deposition directions decrease by 14.1% and 47.5%, respectively, while the differences in elongation decrease by 11.1% and 55.3%, respectively. The static magnetic field can significantly weaken the anisotropy of the mechanical properties of SLM parts since the static magnetic field changes the grain orientation during the SLM process.