WCp/Ti-6Al-4V graded metal matrix composites (MMC) layer was produced by the laser melt injection (LMI) process and the formation mechanism of the graded layer was studied. The results show that the initial speed of the particle (v0), the minimum velocity of injected particles necessary for penetrating through the melt surface (vmin) and the viscosity of the melt pool (η) play key roles in the distribution of the WC particle. With big density, the WC particle has high kinetic energy during the laser melt injection process, and therefore the viscosity of the melt pool is not a critical factor anymore. The solidification front of the solid-liquid interface of the melt pool is the most sensitive factor that determines the formation of the graded layer when the WC particle is used as injection particle. Meanwhile, the total amount of WC particle at different depth of the MMC layer is dominated by the length of the solidification front in this depth. The injection location plays an important role in the distribution of WC particles. When the WC particles are injected into the extended part of melt pool, the depth of this region is low ,and the WC particles meet the solidification front at higher position. In that case, most WC particles are trapped at the top part of the melt pool and therefore a WCp/Ti-6Al-4V graded metal matrix composites layer is formed.