Performance of free-space optical communication (FSO) system fluctuates greatly, which is influenced by atmospheric turbulence. Research about evaluating system error performance according to parameters of system and atmosphere is a subject with certain practical significance. Based on both optical turbulence channel and photoelectric detection model, a mathematic simulation model of error performance for FSO system is established. A modified expression of bit error rate for FSO system through turbulent atmosphere is proposed, and an experiment of 24 hours′ duration is carried out to test the expression. It shows that intensity probability distribution has a significant influence on system error performance. The results of original expression are inconsistent with experimental data on certain conditions, and the modified expression has better applicability and accuracy. The modified expression can lead to an efficient performance evaluation and provide reference to correlative researches.