A novel method (dynamic water transfer–based water activity analyzer (DWT) method) based on Fick’s law of diffusion for the accurate measurement of moisture sorption isotherm (MSI) has been developed and was compared with saturated salt solutions (SSS) method and dynamic vapor sorption (DVS) method. MSIs at 25 °C of sultana raisins obtained by the three methods were analyzed and compared, and four adsorption models (BET, Halsey, GAB, and Peleg) were used to fit the results. The MSI curves obtained by the three methods all showed the similar type III isotherm characteristic, but equilibrium moisture content at the same relative humidity (RH) showed some differences, and the repeatability and accuracy were different. Generally, results obtained by the SSS method may have relatively low accuracy due to the relatively high measurement error; results obtained by the DVS method may lack representativeness due to the small sample size; results obtained by the DWT method may have high representativeness and accuracy at the same time. The fitting results of adsorption models indicated that MSI results obtained by the DWT method had the highest fitting degree with the Peleg model. This study may contribute to deepened understandings on MSI measurement of semi-dried foods.