To improve the performance of infrared focal plane arrays, multi-phase-level 256 × 256 element diffractive microlens arrays with a center wavelength of 4 µm are designed using a novel partial etching method based on the scalar diffraction theory and computer aided design. A testing system is established with an optical communication semiconductor laser and detector for measuring the diffraction efficiency and point spread function of the designed multi-phase diffractive microlens arrays. The measurement results indicate that the measured diffraction efficiency of 16-phase Si and quartz diffractive microlens arrays are 83.7% and 87.5%, respectively, and the optical response of the infrared focal plane arrays with integration of the diffractive microlens arrays is increased by a factor of 2.23.