There has been a surprising improvement in uncooled microbolometer infrared focal plane array(UMBIRFPA) overseas. Compared with other infrared detectors, microbolometer is more promising in commercial use. The research work on UMBIRFPA has been a hot spot in our country. Its physical model and theoretical calculation are introduced in this paper. The thermal equilibrium equation is acquired using the theories of black radiation and heat transfer, and the numerical result is achieved by using MATLAB software. The analysis indicates that thermal conductivity, thermal capacity, bias mode are key points in designing uncooled microbolometer.