Abstract: In order to ensure the accuracy in measurement of atmospheric coherence length, it is necessary to specify qualifications of measurement. The qualifications of differential image motion monitor are determined through analysis of several critical problems such as sampring frequency, spampling time and exposure time in measurement of atmospheric coherence length, and combining with the result of domestic and foreign research, it is ensured that the sampling frequency should not be lower than 100 Hz, the sampling time should not be shorter than 20 s, the exposure time of single frame image should not be longer than the Greenwood time constant along with the measuring beam path, and the number of samples should not be less than 2 000 frames.