By use of the series representation of the Bessel function of the first kind and zero order, the approximate expression of the wave structure function under all conditions of atmospheric turbulence is derived. Based on the extended Huygens-Fresnel principle, the Hufnagel-Valley profile model of the turbulence which is dependent on altitude, and the wave structure function under all conditions of atmospheric turbulence, the mutual coherence function of quasi-monochromatic Gaussian Schell-model (GSM) beams propagating in the slant path through turbulent atmosphere is derived analytically. The longitudinal coherence length (that can be derived from the mutual coherence function) of quasi-monochromatic GSM beams is used to characterize the temporal coherence of them, and the effect of atmospheric turbulence on the temporal coherence of those beams is studied. The result shows that the temporal coherence keeps unchanging in the propagation. Lastly, a qualitative explanation is given to this result.