In-situ integrated microlens arrays are fabricated on the illumination side of the IRFPA(Infrared Focal Plane Array)to increase the signal and suppress the crosstalk,in this paper.By optical analysis,the microlens structures for different size of pixel are carefully designed.With in-situ integrated microlens,the LW(Long wave)320×256 device shows a significant contraction of IR illumination,resulting in a signal increase by 28 %(50 μm pixel)and 61 %(30 μm pixel),respectively.This research has significant meaning for the ultra-high sensitivity IR detection,SWaP,and large format array IR detection.