The temporal pulse broadening of cohered laser pulse caused by atmospheric turbulent and dispersion is analyzed. The expression of turbulence-induced and dispersion-induced pulse broadening is given. Based on the measured data of atmospheric parameters and the fitting formula, the relations of pulse broadening to the zenith angle, original pulse and the wavelength of optical signal are obtained．The background noise in satellite-earth quantum key distribution is simulated. The simulation shows that when the wavelength is 1.06 μm and the original pulse is 15 fs, the final pulse has the minimum. For different zenith angle, the background noise is about 1.9×10-10~2.7×10-10, and the quantum error bit rate is about 0.5×10-5.