Angular separation of real zenith angle beacon and project laser due to atmospheric dispersion is calculated in this paper. Several factors, such as weather, site, project zenith angle and beacon wavelength, are under consideration. Project zenith angle and beacon wavelength have much influence on the angular separation. Weather and site have relatively less influence on the angular separation. The larger the project zenith angle is, the larger the angular separation is. Beacon wavelength should be close to the wavelength of the project laser and longer wavelength is profitable.