Abstract: A dual-beam laser Doppler velocimeter (LDV) with Janus configuration was put forward, in order to reduce the effect of vehicles vibration on measurement accuracy of the conventional dual-beam LVD. Theory analysis and simulation of the relative error of measurement show that the pitch angle can be calculated approximately by Janus configuration technique, and then the velocity can be compensated. The precision of the conventional dual-beam LDV is much affected by the pitch angle, whereas the Janus-based system is relatively insensitive to it. In Janus-based system, the relative error of velocity measurement is less than 0.2%, when the ratio between vehicle velocity in vertical direction and that in horizontal direction equals to 0.01 and the pitch angle is smaller than 10.