The Differential Image Motion Monitor (DIMM) is a widely used instrument for measuring atmospheric coherence length. The miniaturization of the instrument is limited by the requirement of receiving sub-aperture in the traditional formulae. Through theoretical analysis and numerical calculation, the physical meaning of formulae was clarified, and the new calculation formulae were proposed to reduce the limitation on the geometrical structure of the instrument. A miniaturized DIMM test prototype was obtained by replacing the mask in the original standard DIMM with two adjacent 6 cm sub-aperture mask. The miniaturized DIMM prototype was used together with a standard DIMM to simultaneously observe the same star for the comparison experiment of atmospheric coherence length measurement. For miniaturized DIMM, the experimental results show that the longitudinal atmospheric coherence length obtained by the traditional formulae is significantly larger than the transverse, and the longitudinal and transverse atmospheric coherence length obtained by the new formulae is more statistically consistent. Thus, the discussion on the calculation formula in DIMM is verified.