The laser power attenuation by powder flow in coaxial laser cladding is investigated numerically and experimentally. A steady model of powder concentration distribution is developed, considering the effect of substrate on gas-flow and rebound of powder particles. The relationship between powder concentration and attenuation of laser power is analyzed. The effects of melt pool size and powder flow rate on a top hat laser beam attenuation are investigated. Results indicate that power attenuation by powder flow with the effect of substrate can be twice more than that without substrate. The attenuation is proportional to the powder flow rate and decreases with the increase of melt pool size when the pool size is small.