In the Terahertz wall detection system, a detection method based on Terahertz wave is presented to solving the existing problem that due to the crosstalk of reflected signals of adjacent steel bars when multiple steel bars are side by side. The problem of overlapping influence measurement is caused by Terahertz beam illuminating two adjacent steel bars, which is solved by installing a focus beam with adjustable lens group in front of the Terahertz source. What is more, the improved algorithm is used to detect the side-by-side steel bars buried at different depth. The results reveal that the measurement error is less than 0.5mm when the cement depth is 40mm and the distance between two side-by-side steel bars is 25mm. The measurement error is also less than 1mm when the adjacent steel bars have a depth difference of 40mm.