The stress-fractional plasticity model has been proposed to model the state-dependent stress-strain behavior of granular soils. However, the previous model was established based on triaxial test, which cannot be easily used to predict the three-dimensional (3D) stress-strain characteristics of rockfill. To solve this problem, an attempt is made to modify the fractional plasticity model for rockfill by using a novel concept of characteristic stress to make the model capable of predicting the 3D stress-strain behavior of rockfill. To validate the proposed model, a series of true triaxial test results of rockfills under different initial states are simulated, from which a good agreement between the model predictions and the corresponding test results is found. Comparing with the classical plasticity model, no additional plastic potential is required; in addition, the model can describe 3D strength characteristics by mapping from the characteristic stress space to the normal stress space, where no specific 3D strength criterion is needed.