To solve the problem of wear failure of 42CrMo steel and meet its application requirements, in situ NbC-particle-reinforced composite coatings were prepared on a 42CrMo steel surface using laser cladding with a mixed powder (Co-based, Nb,and Cr3C2 powder). The effects of NbC content on the microstructure, wear behavior, and NbC morphology of the composite coatings were analyzed. The results show that when the mass fraction of NbC is 0--15%, the coating bonds well with the substrate and no obvious defects are observed in the coating. When the mass fraction of NbC reaches 20%, microcracks appear in the coating. The matrix of the composite coating primarily comprises ε-Co and γ-Co. The primary strengthening phases are NbC, Cr23C6, and Cr7C3. NbC particles are formed by the in-situ reaction of Nb and C atoms that are dissolved in the molten pool. With the increase of NbC content, the shape of the NbC particles gradually changes from quadrilateral to petaling. The microhardness and wear resistance of the composite coatings clearly increase, and the primary wear forms of the composite coatings are abrasive wear and hard-phase spalling wear. When the mass fraction of NbC is 10%, the microhardness and wear resistance of the coatings attain their maximal values of 546.4 HV and 0.020 g/min, respectively.