To elucidate the deformation mechanisms and mechanical behavior of FGH96 superalloy under high-temperature multiaxial fatigue conditions relevant to gas turbines, tension–torsion combined fatigue experiments were conducted at 650 °C. The cyclic stress evolution under uniaxial fatigue, 0° proportional loading fatigue, and 45° and 90° non-proportional loading fatigue was systematically investigated. EBSD and TEM were employed to examine the deformation mechanisms of FGH96 superalloy under both proportional and non-proportional loading conditions. Based on the Chaboche viscoplastic framework, a unified constitutive model incorporating non-proportional hardening was developed. The results indicate that FGH96 superalloy exhibits pronounced non-proportional hardening under non-proportional loading. The additional hardening mainly arises from intense dislocation multiplication and interaction, accompanied by severe dislocation entanglement. With increasing degree of non-proportional loading, the dislocation density gradually increases, and slip lines evolve from straight to curved configurations. Compared with the traditional Chaboche viscoplastic unified constitutive model, the proposed model incorporating non-proportional hardening significantly improves the prediction accuracy of the hysteresis loop stress range, achieving relative errors below 5%, which satisfies typical engineering requirements for deformation prediction.