Aiming at the question of the short distance for laser Doppler velocimeter（LDV），an idea of using the optical structure of a similar telescope to transform the Gaussian beam is proposed. The principle of velocity measurement with laser Doppler and the specific method of Gaussian beam transformation ware expounded while a portable decameter level laser velocimeter is designed. The result of theory and experiment show that it could acquire the speed of the passing vehicle with the characteristic of portable. The working distance of laser velocimeter is from 6 to 14 meter while the measurement accuracy is 0.2%. So it is achievable to detect the overdrive using the portable decameter level laser velocimeter in the expressway.