Nickel (Ni)/tungsten carbide (WC)/graphene oxide (GO) (Ni/WC/GO) composite cladding fusion coating was fabricated using vacuum cladding technique. The microstructural appearance characteristics and corresponding composition of the whole composite coating were observed under different temperatures conditions and analyzed through scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometer. The forming process and mechanism of composite cladding fusion coating were also investigated systematically. Results show that four zones, namely, approximately 1.5 mm thick composite zone, about 300 μm thick metallurgical fusion transition zone, around 30 μm thick diffusion–fusion zone, and roughly 400 μm thick diffusion–affection zone, are found from the coating surface to the substrate. The main phases of the Ni/WC/GO composite cladding fusion coating are Cr7C3, FeNi3, WC, Cr23C6, Ni3Si, C, Fe7W6, and γ-Ni solid solution. In the composite zone, flocculent complex phase carbides that contain W, Cr, and Ni were formed. The diffusion–fusion zone was mainly composed of Ni-based solid solution and eutectic structure, whereas the diffusion–affection zone was mainly composed of pearlite. The phase size of the composite zone is smaller than that of the transition zone. During the formation of composite cladding fusion coating, the surface melting and fusion of Ni-based alloying particles in a certain thickness zone on the composite prefabricated coating surface were all prior to the transition zone. The formation of composite cladding fusion coating mainly depended on the formation, growth, wetting to WC particles, and fusion of the sintering neck, until to completely fusing of all the sintering necks each other in prefabricated layer to form the composite cladding fusion coating. © 2020 Elsevier Ltd