Atmospheric effects on links limit the application of free space optical communication technology. Researches of mechanism and mitigation technologies of atmospheric effects help to decrease the bit-error-rate(BER), enhance the data rate and available distance of optical communication. The mechanism researches of atmospheric effects on free space optical communication were briefly summarized, and the various influences of atmosphere on that were described, such as intensity attenuation, optical pulse stretch, intensity scintillation, beam wander, beam spreading and phase fluctuation, etc. The existing mitigation technologies of atmospheric effects were summarized. Some technologies were described, such as large aperture receiver, multiple wavelength propagation, multiple beam propagation, partially-coherent-beam propagation, adaptive optics, etc. Lastly, future research directions were given.