Abstract: Nowadays, the X-ray machine is developing rapidly in nondestructive testing, but the distance between the plate surface and the real imaging surface was less taken into account. There are however, some errors in the measurement object distance, thus affecting the calculation of image unintelligibility. This article first measures the minimum resolution of X-ray machine through a standard step test block. Through the experimental results of the standard test block, the appropriate parameters such as electric current and voltage are selected to transmit some certain block. Then according to the theoretical formula the distance of the plate surface and the real imaging surface is calculated. So we can get more accurate object distance in the following nondestructive testing and have an important significance of calculating the unintelligibility of the image.