Based on the rate equation of semiconductor laser, the expression of laser nonlinear intermodulation distortion was deduced by Bessel function. According to the GammaGamma model under the turbulence channel, the influence of performance about carrier signal to intermodulation distortion and noise ratio and bit error rate was researched based multiple subcarrier binary phase shift keying modulation.The selection of optimal modulation index with the corresponding relation of nonlinear coefficient was given. The results show that under the same turbulence condition, carrier signal to intermodulation distortion and noise ratio increases and bit error rate decreases with the increasing of modulation index when the value of modulation index is less than the optimum index. However, when the value of modulation index is bigger the best index, system performance deteriorates. When the scintillation index is 0.2 and the optimum modulation index is 0.6, the maximum carrier signal to intermodulation distortion and noise ratio can get to 12.9 dB.