Free space optical communication(FSO) is a communication system that realizes two-way information transmission through laser without optical fiber. It has the characteristics of wide bandwidth, good confidentiality, strong anti-interference, no frequency application, small size and low power consumption. Despite the high security of the FSO communication system, beam overflow still threatens the security of the communication. The use of chaotic encryption in communication can effectively improve the security of the system. In this paper, the differential chaos keying(DCSK) Gamma-Gamma turbulence model was used to improve the security of the FSO communication system, the expression of the system bit error rate was derived. Through the image data transmission of spatial channel, and the statistical histogram, pixel correlation and plaintext sensitivity were analyzed on the received data, the results show that the system has good security performance.