Friction stir incremental forming (FSIF) is a practical forming technology for hard-to-form materials without additional devices or changes to the equipments. Temperature of sheet metal has an important influence on its formability in FSIF. Rosenthal model with line heating source used frequently in welding was developed to predict the temperature of sheet in this work. The relationship between the effective thermal power and the processing parameters was obtained using central composite response surface experimental method, with AZ31B magnesium alloy as the experimental material. On the basis of this relationship, the temperature model was established. Furthermore, the comparison between calculation results and experimental results show that the developed temperature model can be used to predict the temperature of sheet in FSIF. In addition, the influence of processing parameters on temperature of sheet metal was analyzed based on this model.