A new three-dimensional laser Doppler velocimeter (3D LDV) by only using frequency shift separation method has been developed. Simultaneous measurementS of three velocity components are realized by a single color He-Ne laser and one photodetector with simple transmitting and receiving optics. The signals corresPOnding to different velocity components are distinguished by three different optical frequency shifts introduced by three Bragg cells, and separated by band-pass filters. A novel and simple four-beam transmitting optics which contains three sets of dual-beam LDV systems is proposed, and the design specification of the 3D LDV system is presetned. The measuring accuracy of the on-axis component is also discussed in this paper.