Abstract: Droplet velocity is one of main factors influencing the coating properties.Based on the theories of gas dynamics and multiphase fluid mechanics,the velocity problems of the atomization gas and droplet during high velocity arc spraying(HVAS) were modeled and solved by a numerical method,and experiments were carried out to measure the average velocity changes using Al and 3Cr13 wires.A good agreement between the numerical and experimental results was observed.The results showed that the exit velocity of atomization gas maintained the original velocity(approximately 690m/s) for a certain distance,then decreased with increasing distance from the nozzle,and this corresponded to the interaction between expansion wave and compression wave after the supersonic velocity gas passing the Laval tube.The in flight droplets experienced an acceleration deceleration velocity profile.The smaller droplets could reach higher maximum velocity at shorter flight distance.Beyond the flight distance where the maximum velocity was reached,the smaller droplets were decelerated rapidly due to their lower inertia force.On the other hand,the larger droplets did not exhibit a marked deceleration because of their high inertia force.And the droplet velocity was determined by Reynolds number.Simulation and measurement proved that the maximum velocities of Al and 3Cr13 droplets were greater than the velocity of sound during high velocity arc spraying.