Bit error rate performance of free space optical (FSO) communication systems based on differential detection with zero decision threshold (DDTZ) under moderate and strong atmospheric turbulence is investigated. Computational expressions of average bit error rate of the DDTZ FSO communication system and the differential detection system with optimal threshold are respectively derived when the two transmission lights are completely correlated or uncorrelated. Simulation results show that the problem of the error floor which is presented in single detection with fixed threshold (SDTF) dose not exist in DDTZ. The correlation of two receive signals has no effect on bit error rate of DDTZ. The bit error rate performance of DDTZ is worse than that of optimal differential detection, but better than that of SDTF. Therefore, considering of realizability and bit error rate performance, the DDTZ has more practical application value.