Abstract: This paper establishes a direct，fast method and experiment process of evaluating the effect of the inhibitor for anti swelling by using X ray diffraction instrument — X ' Pert Pro，taking Na smectite for example which has the strongest swelling capacity，making use of the principle of testing the lattice spacing of smectite by X ray diffraction instrument.It systematically tests the changes of Na smectite treated by the distilled water，organic and inorganic inhibitors for anti swelling of clay，shown by the research，the changes of smectite's lattice spacing can directly reflect the effect of the inhibitor for antiswelling。And the author takes two different methods to evaluate the stability and durative of the inhibitor for anti swelling，increasing the performance parameter of evaluating the inhibitor for anti swelling．

Key words: smectite, attice spacing, clay mineral, expansion, Xray diffraction, inhibitor for anti swelling