In order to have a thorough knowledge of characteristics of atmospheric coherence length measured by using differential image motion method, differential image motion monitor (DIMM) is studied deeply. Based on theoretical analyses and numerical simulation, several important problems existing in DIMM are discussed, and their solutions are elucidated. Problems include formulas, sub-pupil diameter and displacement of sub-pupils. Accuracy of DIMM is improved, requirements of instrument structure condition are reduced, and suitable scope of DIMM is expanded.