Moisture adsorption isotherm of freeze-dried scallop, with and without maltodextrin addition (mass fraction of 5% of total solids content), was determined at 25℃ by the gravimetric method. Differential scanning calorimetry was used to determine the glass transition temperature (Tg) and freezing point (TF) of samples equilibrated at various water activities. The state diagram was composed of the glass transition line and freezing curve, which were fitted according to Gordon-Taylor equation and Clausius-Clapeyron equation, respectively. The results showed that the adsorption behavior of scallop displayed J type isotherms. The GAB model satisfactorily described the adsorption behavior of scallop. The equilibrium moisture content in dry basis (EMC) of scallop samples containing maltodextrin (PS-MD) and maltodextrin-free scallop (PS) increased with increasing water activity (aw). EMC of PS-MD at a given aw was lower than that of PS especially at aw higher than 0.62. The monolayer moisture content decreased with maltodextrin addition and the values were observed to be 0.0822 and 0.0716g/g for PS and PS-MD, respectively. Tg decreased with increasing moisture content. Tg decreased from -4.25℃ to -60.92℃ as moisture content of PS increased from 2.81% to 21.96%. Tg decreased from 1.32℃ to -51.41℃ as moisture content of PS-MD increased from 2.11% to 19.72%. TF decreased with increasing solid content. The state diagram yielded maximally-freeze-concentrated solutes at 76.7% and 82.0% solids with the characteristic temperature of glass transition being -70.99℃ and -54.58℃ for PS and PS-MD, respectively. The corresponding un-freezable water for PS and PS-MD were 23.3% and 18.0%, respectively.