The velocity of gas flow field is measured by using Fabry–Perot etalon to detect the Doppler frequency shift of molecular scattering. The working principle of velocity measurement system is introduced. The effects of performance of Fabry–Perot etalon and change of environmental temperature on the velocity measurement are analyzed. The multi-point velocity measurement system is set up. The unshifted incident laser signal and the shifted Rayleigh scattered signal are recorded simultaneously in order to eliminate the effect on the velocity measurement when the positions of interference fringes are changed due to the change of environmental temperature. A couple of large aspherical lens are used to enhance the taking intensity of Rayleigh scattering signal. Through high precision data transform means, the multi-point velocity results of high speed cold crossflow are obtained. The research results show that this method has the capability for velocity measurement of gas flow field with high dependability.