In recent years, lidar system has become a very successful tool in environmental exploration and remote sensing of the atmosphere. However, geometric form factor, which is inherently determined by the lidar structure, restricts the accuracy of the lidar data at nearby distances. In order to get the effective atmospheric parameter information close to the ground from lidar system, it is essential to obtain its geometric form factor. The ratio of the energy received by the photo detector to the energy reached the telescope primary mirror is defined as geometric form factor, which is affected by three facts. First, the overlap of the transmitted beam with the receiver system is often incomplete, so only a part of the return signal goes into the receiving telescope. Second, the backscattering signals from small and medium distances can not be focused well on the focal plane, so only part of them can be sensed by the detector. Third, the obstruction of the secondary mirror can also increase the light loss. By analyzing these three facts, we described a geometric optical calculative method for determining the geometrical form factor in a Cassegrain telescope system. By reviewing the structure of the coaxial and biaxial transmitter and receiver system, and considering the above three reasons, a simple model is applied to demonstrate the image formation of a circular object of diameter G positioned a distance R close to a lidar detection unit. Then the position between the illumination e of the focal plane and telescope aperture s is discussed, and a function to describe the geometrical form factor can thus be derived in both coaxial and non-coaxial lidar cases. Finally, two different lidar systems are compared with simulation method in order to validate the proposed model.