Mechanism of hexadecyl trimethyl ammonium bromide (CTMA) modifying the surface of dodecyl dimethyl betaine (BS-12) modified bentonite was studied. Judging by changes in the Sum of CTMA and Calcium ion (S_CC) on the surface of BS-12 modified bentonite and CTMA adsorption as affected by temperature, 20℃or 40℃, it was found that there were two modes of CTMA adsorption on BS-12 modified bentonite: ion exchanging and hydrophobic bonding. On the surface of BS-12 modified bentonites (25BS, 50BS and 100BS) varying in modification degree, 25%, 50% and 100%, the adsorption of CTMA in the mode of hydrophobic bonding reached its critical level in proportion (R _C), that is, 20.30%, 11.56% and 2.00%, respectively, and the percentage of hydrophobic bonding increased with increasing CTMA modification ratio (R ) and mole fraction. The critical ratio (R _C’) reached 200%, 150% and 100%, respectively, when hydrophobic bonding assumed its absolute dominancy. On the surface of bentonites modified with BS-12 only or with BS-12 and CTMA, 50% and 200% was the turning point for hydrophobic bonding to appear and to assume absolute dominancy, Separately. When R < R _C, CTMA adsorption was mainly in the ion exchanging mode; when R _C ≤ R ≤ R _C’, it was in both modes, ion exchanging and hydrophobic bonding; and when R > R _C’, it was overwhelmingly dominated with hydrophobic bonding mode. R _C、R _C’ and the maximum adsorption of CTMA (q_m) displayed an order of 25BS>50BS>100BS. With rising temperature, q_mdeclined.