In free space optical communication (FSO), optical signals transmit in the open air link. Inevitably, received signals are influenced by atmosphere disturbance and background noise, so the reliability of FSO system is reduced. In order to restrain adverse effects of atmosphere disturbance, a new modulation method of optical signal in FSO, which is named as circle polarization shift keying (CPOLSK), is proposed in this paper. In this modulation, two rotation states of circle polarization are used to denote “0” and “1” data , respectively, then the information transmission accomplished by transmitting rotation state time series. In the receiver, the signal is detected with difference method. The setting of CPOLSK is presented and the performance is analyzed. Compared with on-off-keying(OOK)and pulse position modulation(PPM) modulation methods, which are widely used in wireless communication systems, the CPOLSK modulation has more excellent capability in background noise suppressing and photodetector inherent noise limiting.