As the main technology of defect nondestructive testing, radiographic testing has unique advantages in defect characterization, but it is difficult to measure the size of the defects along the X-ray penetration direction. For this reason, taking steel plate as the research object, based on the radiographic imaging theory, the relationship between the image gray and the penetration process conditions, linear attenuation coefficient, and penetration thickness was studied. Numerical integration and curve fitting methods are used to convert the linear attenuation coefficient into tube voltage and penetration thickness expressions. The final image grayscale-penetration thickness relationship model is obtained. By using the experimental platform to obtain the experimental data, the unknown parameters in the model are obtained by software fitting, and the accuracy of the model is measured by the steel step test block, and the actual thickness and the calculated thickness of the step test block are compared. The results show that the actual thickness and the calculated thickness are close, the error range is 0.18%--1.49%. The experimental results prove the accuracy of the model well, and the thickness of the defect in the steel material can be calculated according to the actual penetration conditions and the image gray level, and the size of the defect along the ray penetration direction can be obtained. Combined with the digital imaging image to measure the size of the defect perpendicular to the ray direction, the defect can be evaluated in an all-round way.