By using the mutual coherence function of quasi-monochromatic Gaussian Schell-model (GSM) beams propagating in a slant path through the turbulent atmosphere, the complex degree of coherence of those beams is derived analytically. By employing the lateral coherence length (that can be derived from the complex degree of coherence ) of quasi-monochromatic GSM beams to characterize their spatial coherence, the effect of atmospheric turbulence on the spatial coherence of the beams was studied. The result shows that：(1) When θ≤88°, the lateral coherence length increases firstly and then keeps unchanged as the beam propagates in the turbulent atmosphere. (2) When θ≥89°, the lateral coherence length increases firstly and then reaches a maximum value after the beam propagates acertain distance in the turbulent atmosphere, then begins degrading and keeps decreasing with increasing distance. (3) Three beams with different lateral coherence lengths at the source plane (z=0) will have almost the same lateral coherence length after they propagate a certain distance in the turbulent atmosphere. Lastly, a physical explanation was given to the results.