To repair the surface fatigue crack defects of WC-Ni-Co cemented carbide and characterize the influence of different preheating temperatures on the microstructure, microhardness and friction and wear properties of the laser melting layer, this paper prepares the WC-Ni-Co cemented carbide melting layer with different preheating temperatures by 4 kW fiber laser, detects the surface cracks with the coloring flaw detection agent, observes the microstructure of the laser melting layer with optical microscope (OM) and scanning electron microscope (SEM), measures the element distribution and phase composition of the laser melting layer by the energy dispersive spectroscopy (EDS) and X-ray diffractometer (XRD), and its hardness and wear resistance through the microhardness tester and wear tester, as well as analyzes the wear morphology and wear mechanism of the laser melting layer. The laser melting layer contains four structures:the original WC phase, the α-Co matrix phase, the eutectic fishbone carbides and the dispersed fine secondary carbides. Compared with the irregular shape of the substrate WC particles, after the surface melting, the WC particles grow up significantly, the interface is flattened, and the eutectic fishbone carbide is a mixture of WC, Cr7C3, CoCx and C6(CoCrNi)23. The range of the laser melting layer gradually increases with the increase of the preheating temperature, which is up to 866.7 μm. When the preheating temperature reaches above 400 ℃, there are no cracks on the laser melting layer obtained after melting. The average microhardness of the laser melting layer reaches 934HV0.5, which is much higher than the substrate hardness of 762HV0.5. With the increasing of preheating temperature, the friction coefficient of the laser melting layer will decline and the wear resistance of the surface will improve. When the preheating temperature reaches or exceeds 400 ℃, the laser melting layer is featured with uniform distribution of WC particles, no defects such as cracks, and higher hardness and wear resistance.