Differential scanning calorimetry was used to determine the glass transition temperature (Tg) of vacuum-dried yacon powder with (yacon:maltodextrin total solid mass ratio of 1〖∶1) and without addition of maltodextrin (MD). GAB model and Gordon-Taylor equation were fitted to the sorption isotherms and Tg data, respectively. The state diagram was constructed to clarify the effect of MD on the critical water activity (CWA) and critical water content (CWC) of YP and YP-MD. The results showed that Tg decreased with increasing water content. Tg decreased from 15.8℃ to -72.6℃ as water content of YP increased from 5.95% to 30.88%. Tg decreased from 30.5℃ to -53.5℃ as water content of YP-MD increased from 5.11% to 27.66%. Addition of MD significantly increased the Tg of yacon powders. At temperature of 25℃, the CWC and CWA of YP and YP-MD were 0.045 5 g/g, 0.12, and 0.072 3 g/g, 0.27, respectively. Therefore, the addition of MD could significantly increase the CWC and CWA, and then improved the storage stability of yacon powder.