Abstract: In the technology of X-ray digital imaging detection, the X-ray imaging shows changes in gray value for thickness variations along the transillumination direction. The way how to get the object's actual thickness changes by grey value change becomes the focus of the research. In this paper, based on Beer theorem and combining with the factors influencing the intensity of X-ray machine and ray intensity and digital image transformation relations, the quantitative relationship between the grey value and thickness is deduced. By setting up the experimental platform to get series of experimental data, the theoretical calculation value and experimentally measured values are compared to verify. The results show that the difference between theoretical gray value and actual grey value is not more than gray full range of ±10%.