Evaluation for decision threshold corresponding to the minimum bit error rate (BER) of modulating retro-reflector (MRR) free space optical communication (FSOC) system is presented. And based on this threshold, the closed form expression of the BER for MRR FSOC system is obtained, considering the effects of adaptive threshold noise and photodetector noise. According to the derived expressions, the effects of adaptive decision threshold parameters and modulation extinction ratio on BER are analyzed. Results show that the BER of MRR FSOC system with adaptive threshold precedes to that with fixed threshold 17.5 dB (BER is 10 -5, weak turbulence), and when the number of training data bits is greater than 3, the BER performance of the adaptive decision threshold system is similar to the BER performance of the decision threshold system based on instantaneous channel state information.