Atmospheric scintillation of uplink optical beam has the effect on reliability of the satellite-ground laser link. Based on time-averaging effects for atmospheric scintillation, extending photoelectric detector’s exposure time is one of the effective ways to mitigate the extent of atmospheric scintillation. Time-averaging effects of exposure time taken into account, expressions are derived for time-averaging factors for large zenith angles and small zenith angles, respectively. Based on the above, that the zenith angle and height above ground of an optical transmitter have the effect on the time-averaging is analysed. Results show that extending the exposure time mitigate the extent of the atmospheric scintillation of uplink optical beam. Along with the increase of the zenith angle and height above ground of an optical transmitter, the extent of the time-averaging effect decreases.