In order to study the effect of fog on quantum communication in free space, the effects of fog visibility and transmission distance on link attenuation, quantum communication channel capacity, channel fidelity and channel error rate are studied according to the distribution function and extinction coefficient of fog drop spectra. The simulation experiment is also conducted.The results show that when the transmission distance is 9 km and the visibility is 0.2 km and 0.6 km, the corresponding link decay, channel capacity, channel fidelity, channel error rate are 0.76 and 0.25, 0.24 and 0.4, 0.91 and 0.97, 0.01 and 3.26×10 ^-7, respectively. It can be seen that when the visibility of foggy days is less than 1 km, the influence of fog on the performance is more significant. Therefore, the parameters of the communication system should be adjusted to ensure the smooth progress of communication based on fog visibility.