Uncooled infrared focal plane detectors are widely used in military and civilian markets, and microbolometer with small image elements and high absorption is a current research hotspot. The infrared absorption model of a two-layer umbrella structure microbolometer detector unit is designed, and the infrared absorption of the detector unit is systematically analyzed. Based on the optical conductivity matrix method and impedance matching theory, the three-dimensional electromagnetic simulation software CST is used to study the optical absorption characteristics of the umbrella-shaped open-hole model microbolometer. The results show that the optical performance of the double-layer umbrella-shaped open-aperture microbolometer is closely related to the size of the umbrella-shaped structure of the open-aperture. The introduction of openings in this umbrella microbolometer reduces the thermal capacity of the detection cell while maintaining the high infrared absorption characteristics, thus improving the device response speed. The average absorption rate of the detector unit in the 8-14 μm IR band is 85%, which meets the design requirements of the ultra-large scale small pixel uncooled IR focal plane detector.