The radio on free space optical transmission (RoFSO) link is proposed to meet with the large capacity requirement of satellite-to-ground communication. With the established link model, the link optical loss, the output power of microwave signal, noise and the bit error rate (BER) under atmospheric turbulence channel are deduced. The influence of bias voltage of the electro-optic modulator on the signal to noise ratio (SNR) and BER is simulated. The analysis results show that compared with the quadrature bias point, the optimal control of the bias voltage can improve the SNR and BER. And the effect of improvement increases with the decrease of the modulation coefficient. The research gives good foundation for the realization of RoFSO based satellite-to-ground communication system.