Atmospheric coherence length is an important parameter for evaluating the effect of adaptive phase correction in a laser transmission system, so there is a high research value to measure and analyze the atmospheric coherence length. In this paper, a system for measuring atmospheric coherence length by differential image motion method is designed and built to carry out field experiments in Xi'an area, and the variation trend of atmospheric coherence length within the whole day in Xi'an area is analyzed. Results show that although the values of atmospheric coherence length measured in different directions are different, the overall variation trend is the same, and the variation is more stable at night than at day. These results have a reference value for the construction of urban wireless optical transmission systems.