For the measurement needs of space three-dimensional coordinates, a 3D laser radar optical system was designed. This system used the structure of transmitting/receiving common path and based on Gaussian optics theory. It can not only improve system concentricity, reduced external interference, and also simplify the system structure, reduce the volume of the instrument. The system uses expanded beam collimation structure to achieve eight times beam expander. Through small defocus, by changing the distance between 28.203-14.671 mm of the transmission optical system between a first lens and the second, it can achieve ranging accuracy 0.02 mm+10 μm/m when the measuring range of 2-18 m.