Abstract: In order to reconstruct the thickness distribution of the cross-section in the steel strip for the profile gauge using two X-ray sources and two arrays of gasfilled ion chambers, it’s important to compute accurately the positions of the cross-section’s edges first. Based on the practice profile gauge, the computation modeling of X-ray attenuation at the edges of the cross-section was achieved by using the principle of X-ray attenuation in materials. With the detectors’ output and the modeling, the computation of the edges of the cross-section was completed. The result reveals that the computed edge is right the midpoint of the edge and the algorithm for edge computation is accuracy.