In order to meet the need of measuring the train groups’ three-dimensional coordinates, a 3D laser radar common path zoom optical system was designed. According to the characteristics of laser beam expender, multistage laser amplifiers were carried out by using lens chain. Based on Gauss optics and geometrical optics, the optical structure of the laser emission system based on liquid lens was fully analyzed, and the key parameters of the initial structure of optical system were calculated. Using Zemax optical design software to optimize the simulation, the 3D laser radar transmitting/receiving optical system was designed. The spot diagrams of the transmitting optical system which used a liquid lens to zoom were all less than 20 μm/m within the image distance range between 2 m and 30 m, and the spot diagram decreased with the increase of the distance. By minimizing the spot size which was detected by the array detector channel of the receiving system to focus the energy in one point, then the power of liquid lens was the optimal state to make sure the spot diagram in the target was smallest. By optimizing the design, the radius of the 90% energy concentrated area were less than 1.6 μm. The optical structure not only improves the system alignment, reducing the external disturbing, but also simplifies the structure of the instrument volume. Using the liquid lens focusing instead of mechanical focusing to avoid the vibration, and the positioning of the laser radar was proved.