The photoelectric signals of wireless optical communication transmit-receive system which used the intensity modulation direct detection (IM/DD) and on-off keying (OOK) were analyzed. Maximum a posteriori probability (MAP) method was performed to confirm the decision threshold. By employing lognormal distribution as the scintillation model in turbulence channel, the quantity relationship between system bit error rate (BER) and system factors such as turbulence intensity, source coherence parameter, optical power of laser transmitter, etc, was established. Adopting partially coherent light beam as signal source laser diminished the coherence and thus restrained the turbulence effect availably. The calculated results indicate that the behavior of system BER transforms 6 to 8 orders as turbulence intensity changes with a half order under some certain condition. Partially coherent beam with optimum coherence parameter, which can be acquired through computation, is most favorable for enhancing the communication system performance.