Based on the technology of monolayer microbolometer, which used Si3N4 and SiO2 double-layer membrane supporting materials, the analytic expressions of the normal stress and the thermal conductivity of the microbridge leg were derived by using the mechanical equivalent cross-section methods and the composite thermal conductivity formula. The influence of the thickness ratio m of Si3N4 to SiO2 on the mechanical and thermal characteristics of microbridge was analyzed when other factors were not changed. At last, the theoretical derivation was verified by the method of ANSYS finite element simulation.