Optical and thermal designs of microbolometer with four leg microbridge were described. The optical characteristic of multilayer structure consisting of SiO 2 infrared absorber film / poly Si thermosensitive film / SiO 2 support film was analyzed, and the influence of SiO 2 infrared absorber film thickness on multilayer structure average absorptirity was investigated. During thermal design, the average infrared absorptivity of microbolometer is not fixed as a constant but changed with thickness of absorber film, which is different from the conventional method. The optimized thickness of infrared absorber film and length of support leg were achieved through Finite Element Analysis software ANSYS5.7.