In this study rigorous numerical models were utilized to characterize the focal shift of the diffractive mid-infrared (MIR) silicon microlens array (MLA) with the critical size smaller than the working wavelength. We found a more pronounced focal shift in mid-infrared wavelength which is out of the range predicted by existing focal shift model. Focal properties of the MLA were also measured experimentally and shown to follow the simulation results. Our results provide a reference point in understanding the focal shift in MIR regime, which is important in terms of deciding the focal length of micro-nano lens systems, especially when dealing with the integration of diffractive devices in infrared optical system.