In order to measure the longitudinal and lateral speeds of a vehicle simultaneously, a triple-beam laser Doppler velocimeter (TLDV) system based on Janus configuration is presented. The model and the expression of speed measurement are given. According to the analysis and simulation, some results can be obtained as follows: firstly, TLDV is less sensitive to bump and shake; secondly, the relative speed measurement error is affected only by the bump angle and has no relationship with the shake angle; thirdly, the relative error is lower than 0.3% when the bump angle is 10° and the velocity is 0.2 m/s. Last of all, a kind of optimal configuration of TLDV beams is given.