The invention provides a steel structure rust detection device based on microwave thermal imaging. The device comprises a microwave heating device and an infrared thermal imaging device, wherein the microwave heating device comprises a magnetron and a horn antenna, wherein the magnetron is connected with the horn antenna; the horn antenna is towards the surface, to be detected, of the a steel structure; the infrared thermal imaging device comprises an infrared thermal imager and a display device; the infrared thermal imager is connected with the display device; the infrared thermal imager is towards the surface to be detected. A detection scheme of steel structure damage in a protective layer is provided from a new technical angle, the rust condition of a steel structure under the protective layer can be nondestructively detected, and a reinforced concrete structure can be properly managed and maintained.