Abstract: Particle temperature and velocity is critical to the performance of coating sprayed by High Velocity Oxygen/Air Fuel spray system, which is mostly dependent on the flame temperature and velocity accelerated by convergent-divergent nozzle. Gas characteristics of inlet, throat and outlet of the nozzle are calculated. Characteristic diameter and inner shape of the nozzle is designed according to aerodynamics. Ther results show that the velocity of the flame reaches supersonic while oxygen and air is used as combustion-supporting gas respectively. At the same time, the pressure of the nozzle outlet is less than the ambient pressure, which is advantageous to powder feeding. Flame velocity can be continuously adjusted from 1400 to 2150 m/s, while flame temperature is 1400~2600℃. The adjustable flame can meet the requirement of spraying coating for metal, alloy and ceramics.