Abstract: In the profile gauge for hot steel strip constructed with two X-ray sources and two arrays of gasfilled ion chambers, there’re many components and their dimensions are commonly large. So there’re many errors in the machining of the gauge. The errors are cumulated so that the mechanical parameters in the final gauge will be a little bit different from the design values. Based on the practice gauge, the principle how the parameters’ difference affects the accuracy of the gauge was researched. The result shows that the effect will be reduced when the distance between the two X-ray sources is made bigger and the source positions are made higher from the detectors. Meanwhile if the accuracy is claimed to be 0.1%, the parameters’ difference from the design values can be several ten millimeters.