Space free optical communication (FSO) has the advantages of wide bandwidth resources, less susceptibility to attacks, strong resistance to interference, and low losses. The FSO system itself has high security, with the realization of single photon detection technology, the stealer can obtain information without blocking the beam propagation. So using chaotic encryption methods can further improve the security performance of communication systems. This article uses a gamma gamma turbulence model and utilizes the differential chaotic shift keying (DCSK) method of communication systems to improve the system’s resistance to attacks. The theoretical expression of the system error rate under this model is derived, and the security performance of the system is further analyzed. Through experimental simulation, it is proved that the system has better resistance to attacks and lower bit error rates.