[en] The interaction of X-ray between Al plating and U matrix materials, the effect of U matrix's radioactivity to measurement of Al thickness has been discussed. A mathematical model of the relationship between intensity and plating thickness has been advanced, as well as in the same model, U M and U L ray are selected to measure thin and thick sample,respectively. Nondestructive testing method of Al plating thickness on U matrix has been found by measurement micro-area on the working sample using collimating technology,and carrying out Least Square Regressing Analysis for the result of measurement