The change in coherence length of Gauss-Schell model beam propagating in atmospheric turbulence is studied by comparing with propagation in free space. The coherence length change only depends on source parameters in free space while its change in turbulence is governed by source parameters and turbulence. Beam spreading results in an increase in the coherence length in vacuum. For propagation in turbulence, the coherence length increases due to beam spreading over relatively short distance while decreases on account of the enhancement of turbulence over a sufficiently long distance. Thus, the simple analysis of the influence of turbulence on the coherence length is not mature enough. In order to exclude the effect of beam spreading, the ratio of coherence length to beam size is employed. It is found that atmospheric turbulence always leads to the decline of ratio. The explanations, based on numerical simulation, of above-mentioned results are given in this paper.