The velocity of solid-phase particles of debris flows is a key issue for its kinematics and prevention measures. Flume model tests and particle image velocimetry were conducted to get the solid-phase velocity of debris flow. Effects of slurry viscosity, volume concentrations and particle sizes on the solid-phase velocity characteristics were accordingly analyzed by adopting two-phase debris flow model. The results show that the high-concentrated-particle debris flow head gets weak with the increase of slurry viscosity and volume concentration. The relative motion between solid and liquid phase decreases with the increase of slurry viscosity, and it vanishes when the volume concentration is big and the particles are small. The solid-phase velocity of debris flow has a positive correlation with the slurry viscosity and volume concentration, and has a negative correlation with the particle size. Two-phase composition of debris flows affect the force of friction and/or collision between solid-phase particles, and the viscous resistance force, the viscous drag force and the virtual mass force acting on particles, which are induced by slurry. The influence of these forces on the solid-phase velocity needs further study.