Abstract: The principle of atmospheric coherence length measurement via differential image motion measure of the optical wave's angleofarrival fluctuation is introduced. An instrument for the measurement of atmospheric coherence length both in the daytime and at night were described. The statistical characteristics of the atmospheric coherence length in the daytime and at night were analyzed from some primary observation results. It shows that the refractive index structure constant varies with the time in total layer, and the timevarying behavior is basically in accordance with that of the boundary layer, that is, the atmospheric coherence length during the period of sunrise and sundown is much larger than that in the other periods.