The parameters of steel bar (such as position, diameter, etc.) play an important role in the safety of the constructed buildings. A measurement method for position and diameter of the steel bar within the building by using terahertz wave was proposed. The terahertz wave was used as the detection signal to realize these functions, which the terahertz wave had a strong penetrability to nonmetal building material (such as cement, brick, lime, etc.) and had the characteristics of almost total reflection to the metal (steel bar). At the same time, compared with the microwave, the terahertz wave was easy to achieve a very narrow antenna beam, which achieved high detection accuracy. And compared with the magnetic detector, it was not influenced by the external magnetic field environment. Theoretical analysis and engineering test show that this method can meet the requirements of actual engineering application.