In order to better evaluate the building quality and guarantee the building safety, this paper studies the detection of reinforcement in concrete to realize the accurate detection of the thickness of the protective layer and diameter of reinforcement without prior information. Ultra wideband stepped frequency ground penetrating radar and electromagnetic induction detection modules are developed. A relative permittivity estimation method is proposed, and the accurate thickness of protective layer can be obtained by combining radar data. The electromagnetic induction calibration experiments with different diameters and thickness of reinforcement were carried out, and the calibration database was obtained. The measured electromagnetic induction data and the thickness of protective layer are matched with the calibration database to obtain the exact diameter of reinforcement. The experiment on concrete test block shows that the steel bar can be detected without prior information, and the relative error of the thickness of the protective layer is less than 2%, and the diameter of the steel bar is detected without error. The system and method are proved to be effective and accurate, and can provide effective technical support for building quality evaluation.