NiCrBSi+Ta composite coating is produced on surface of a medium carbon steel substrate by laser cladding in order to in situ synthesize TaC particle. Scanning electron microscope (SEM), energy-dispersive spectrometer (EDS) and X-ray diffractometer (XRD) are used to observe and analyze the coating. Morphology, distribution and formation mechanism of in situ synthesized TaC particle are discussed. In spite of high melting point, equiaxial and flower-like TaC particles are still formed by one-step laser cladding. The result shows that the formation mechanism of TaC particle is nucleation and growth, colliding and sintering, and in situ precipitating. The distribution of particles is homogeneous. Most of TaC particles are entrapped at grain boundaries instead of being engulfed by matrices or pushed to the top of the coating due to high convection, high thermal gradient, multidirectional solidification of matrix dendrites and difference between thermal conductivities of particle and matrix. The entrapment at grain boundaries is favorable for the homogeneous distribution of particles. Acicular chromium borides or carbides are found to grow up around boundaries of TaC particles, which will be beneficial for cohesion strength between particle and matrix. High amount of Si element dissolved in in-situ synthesized TaC particles is also found as compared to other carbides such as TiC, which will supply a new method to control the Si content in ceramic particle strengthening materials.