Abstract: By using the characteristic fluorescent X-ray absorption spectroscopy, a new strategy method was developed to measure the mass thickness of ²³⁵U fission target based on beryllium substrate accurately, and the metrical error was also analyzed. It was found that the mass thickness can be effectively obtained through this strategy method. The metrical error of mass thickness is less than 5%. Both the stability of X-ray resource and statistical fluctuation from X-ray measuring process by detection system can affect the accuracy of experiment result，and it is proved that the statistical fluctuation is a main contribution.