The constitutive behavior of the mushy zone represents the stress–strain relationship of solidifying alloys, which is essential for the prediction of hot tearing, residual stress, and distortion during the casting process. To determine the constitutive behavior and rheological properties of solidifying Al alloy mushy zones, an integrated method that combines a phase field model with a coupled Eulerian–Lagrangian (CEL) approach was developed. The morphologies and evolution of the mushy zone microstructure were first obtained through phase field calculation. Then, the geometric model and corresponding mesh were generated based on the microstructure slices. Finally, finite-element analysis using the CEL method was adopted to obtain the stress–strain relationship of the solidifying Al alloy mushy zone. The rheological properties were derived from the obtained constitutive relationships. A comparison between the predictions for Al–4.5 wt% Cu and experimental results indicates that the proposed method is reliable for the prediction of constitutive behavior.