The relation between equilibrium moisture content and water activity of tough biscuit at 23 ℃, 30 ℃ and 45 ℃ was studied by gravimetric method. The test data were fitted with typical moisture absorption models. The results showed that the moisture isotherm of tough biscuit was S type model; the equilibrium moisture content increases with temperature increases, which was the inversion of other foods; the GAB model is the most suitable for fitting adsorption isotherms of biscuit in the aspects of activity scope and fitting precision.