The average bit error rate (BER) and outage probability of circular polarization shift key (CPOLSK) modulation under the combined effect channel of atmospheric turbulence and pointing errors are investigated by using aperture averaging technique. The closed expressions of BER and outage probability are derived by using Meijer G function. The influences of normalized beamwidth, normalized jitter and receiver aperture size on the average BER and outageprobability are simulated, and numerical analysis is also carried out. And then the BER performance of CPOLSK modulation is compared with that of on-off keying (OOK) modulation. All numerical simulation results are verified by Monte-Carlo simulation. Results show that the BER performance of CPOLSK modulation is better than OOK modulation under the same conditon, and the influence of normalized beam width on system performance is nonlinear and there is an optimal value. It is also shown that with the increase of the normalized jitter standard deviation, the performance of the system will decrease, while increasing the receiving aperture size can effectively improve the performance of the system. The results are of practical significance for the design of FSO communication system.