Abstract: The instantaneous profile gauge uses two X-ray modules and two arrays of gas-filled ionization chambers to measure the thickness, crown and other parameters of steel strips. The energy and intensity of X-ray are the main factors that affect the measurement accuracy. In order to ensure the accuracy, it was assumed that thickness error caused by statistical fluctuation should be less than ±0.06% according to GB/T 15636—2008, the theoretical maximum relative standard deviation of the detector signals was estimated at 180 kV X-ray tube voltage and 11 mA X-ray tube current, some steel strips were measured included in the gauge thickness range, and the relative standard deviation was calculated for each detector. The result indicates that the real statistical fluctuation can be less than the estimation value by averaging several raw data. So the set values of X-ray tube voltage and tube current of the X-ray module can meet the requirement of thickness accuracy.