Free space optical communication system is competent for its large bandwidth, the agility and short fixing time. The bandwidth is affected by many factors, and the optical system is one. This paper firstly introduces the FSO system and the constitution of its optical system, and then analyzes the impact of the lens dispersion on system communicating bandwidth. On the basis of the bandwidth limitation discussion, this paper puts forward a means to use a couple of cemented-doublets group as the transmitting lens, and use the dense whorl lens as the receiving lens. In the end, the use of such design in our own system proved the feasibly of it.