In additive manufacturing, accurately simulating mesoscale microstructure evolution is crucial for predicting the performance of sintered components and optimizing processing parameters. Sintering, driven by both viscous and solid-state mechanisms, governs the densification and shrinkage of powder-based systems—a challenge that traditional sharp-interface models struggle to overcome, particularly in capturing complex morphological transitions and deriving the underlying thermodynamic driving forces. We introduce a novel computational framework that integrates phase-field methods with micromechanics to simulate sintering processes in additive manufacturing. By employing a unified energy formulation and variational principles, our approach derives governing equations that model microstructural evolution with driving forces explicitly expressed in terms of surface and grain boundary tensions. This thermodynamically consistent formulation not only ensures evolution toward minimized free energy but also overcomes key limitations of sharp-interface techniques. Validation against classical analytical solutions and experimental measurements confirms the model’s accuracy. The framework holds significant promise for optimizing sintering-based additive manufacturing processes and may be extendable to a broader range of materials and sintering conditions.