The background of atmosphere turbulence on free space optical sub-carrier modulation is set. The expressions of the cyclic auto- correlation function and the cyclic spectral density of quadrature phase shift keying (QPSK) signals are derived and the cyclic auto- correlation function and the cyclic spectrum density of series of experimental QPSK data are analysed. The results show that both additive and multiplicative noises in the experimental data are influenced by the atmosphere turbulence. And the turbulence which is not very strong can not disturb cyclostationarity of QPSK signals. The main factor of received signal to noise ratio is the power difference between the signal and low frequency noise. By adding a time window which can restrain low frequency noise, the signal to noise ratio of free space optical subcarrier modulation system can be improved.