The mechanism for improving performance of infrared detector using micro optical method is studied. A germanium microlens array was coupled to a 180 unit infrared HgCdTe focal plane array detector. The detectivity of the coupled device was increased to 2.8-fold. As an objective standard, the optical coupling efficiency is proposed to evaluate the quality of micro optical concentrator and the coupled method. The cooling shield effect caused by microlens array, which is useful to limit the background radiation was also discussed.