Abstract: In the X-ray profile gauge for hot steel strip constructed with two X-ray sources and two arrays of gas-filled ionization chambers, one of the crucial factors that affects the accuracy of the gauge is the relative positions of the two X-ray sources, the two arrays of the detectors and the steel strip. Based on the practice profile gauge, the principle how mechanical parameters affect the accuracy of the gauge was researched by combining source angle and noise analysis. When the demands of accuracy and resolution are given and the angle of the X-ray source is known, the optimal parameters can be got by analyzing the deduced principle. In the practice gauge, the method was successfully used.