Abstract: [Objective]The development of deep, weakly cemented heavy-oil reservoirs is typically accompanied by pronounced geomechanical effects. Therefore, an accurate understanding of the mechanical responses of reservoir rocks under different stress states, together with a reliable characterization of their strength, volumetric deformation, and compaction behavior, is essential for development planning and geomechanical evaluation of such reservoirs.[Methods] In this study, core samples collected from the target block were subjected to high-temperature and high-pressure triaxial shear and isotropic stress tests. The mechanical responses of the rocks under different stress states were analyzed. A state-dependent stress–dilatancy elastoplastic constitutive model was then developed by incorporating equivalent cohesion, cementation evolution, stress-state dependence, and a cap yield criterion. [Results] The results showed that the load-bearing mechanism of weakly cemented heavy-oil reservoir rocks progressively shifted from being governed by cementation and temperature to being dominated by interparticle friction and compaction. When the temperature reached 200 °C, the uniaxial peak strength decreased by approximately 77%. Once the confining pressure exceeded 3 MPa, the temperature effect weakened markedly, and confining pressure became the dominant factor. As the confining pressure increased from 3 to 15 MPa, the average maximum volumetric compression increased from 0.66% to 3.69%, while the transition from compaction to dilation was progressively delayed. Under intermediate confining pressures, the rocks were prone to dilatant hardening, whereas compaction dominated under high confining pressures. During isotropic stress cycling, both the maximum volumetric strain attained during loading and the residual volumetric strain after unloading accumulated with increasing maximum prior pressure. Moreover, elevated temperatures promoted substantial irreversible structural rearrangement at relatively low pressures. Model validation yielded median goodness-of-fit values of approximately 0.87 for stress and 0.91 for volumetric strain, demonstrating that the model effectively captures the temperature sensitivity, compaction–dilation transition, and path-dependent deformation of these rocks. [Conclusion] This study provides an experimental basis and a constitutive framework for optimizing development parameters and conducting geomechanical assessments of deep, weakly cemented heavy-oil reservoirs.