Abstract: This paper investigates the effects of medium entropy powder CoCrNi on the morphology and microstructure properties of aluminum/steel TIG brazing joints by coating and prefabrication. The results show that, when the side groove of the steel is coated with medium entropy alloy powder, there is an ellipsoidal unmelted CoCrNi particle layer that can be observed on the brazing surface. The gaps around the particles are filled with gray Al matrix and Al0.7Si3Fe0.3, and there is no obvious elemental diffusion of Al and Fe. The residual CoCrNi particles play a role of enhancing the phase to a certain extent, but the inhomogeneity of their size and distribution affects the flow of the liquid AlSi5welding wire, which leads to poor coverage of the back welding seam, with high weld reinforcement and unfused defects.Under the prefabricated condition, the CoCrNi powder melts and bonds well with the steel interface. Moreover, the fused metal spreads well on both aluminum and steel surfaces, and joints form well. With the interdiffusion of elements, an intermetallic compound (IMC) layer composed of Al0.7Si3Fe0.3,Co3Fe7, Fe(Cr, Ni) and Al0.9Ni1.1forms at the interface, with an average thickness of 3.15 μm. CoCrNi powder significantly reduces the formation of brittle IMC such as Fe-Al phases.Therefore, the tensile strength of the two methods is greater than that of the joint without adding medium entropy alloy powder, and the strength of the prefabricated medium entropy alloy powder is as high as 86 MPa. The maximum Vickers hardness of the joint coated with medium entropy alloy powder reaches 467HV, which is located on the side of the brazing interface near the weld. However, there is a softening zone on the steel side of the CoCrNi under the prefabrication mode, and the highest hardness Vickers is located on the steel side near the brazing interface, which is up to 248 HV.