Desiccant is often used in package of foodstuffs and medicaments to prolong preserving time. Silica gel and montmorillonite commonly used in food packaging were taken as objects of study. The relationship between water activity and equilibrium moisture content were studied by gravimetric method. The test data was fitted using moisture sorption isotherm model. The results showed that the moisture isotherms of silica gel and montmorillonite desiccant were the Ⅰ type and Ⅱ type curve respectively; the Langmuir and Oswin models are the most adaptable for fitting the adsorption isotherms of biscuit silica gel and montmorillonite desiccant respectively; the capacity of moisture adsorption of silica gel is weaker than that of montmorillonite when the water activity is less than 0.32, but it turned to the opposite at high water activity.