Atmospheric coherence length is a very important parameter in the study of atmospheric turbulent intensity and laser propagation. To figure out the A-class measurement uncertainty of the atmospheric coherence measurement system, and confirm the reliability of the difference image motion method (DIMM), the atmospheric coherence length was measured by DIMM and turbulent integral method in the same propagation path. Results of both methods are similar in level and the time trend, so both methods can validate each other. The possible reason of atmospheric coherence length differences between two methods was analyzed. The A-class measurement uncertainty of the atmospheric coherence measurement system was calculated by Bessel method. The average relative extend measurement uncertainty of the system is smaller than 20% via the experiment. The results show that the atmospheric coherence measurement system can offer valuable references for the application of atmospheric optics propagation.