High-temperature spalling of concrete structures subjected to fire exposure poses a serious threat to the safety of underground and civil infrastructure. The occurrence and severity of spalling are strongly influenced by thermal boundary conditions and heating rates; however, their roles in governing fracture evolution remain insufficiently understood. In this study, a fully coupled thermo–hygro–mechanical phase-field framework is developed to investigate spalling behavior of concrete under elevated temperatures. The proposed model incorporates heat conduction, moisture transport, and mechanical response within a unified variational formulation, while crack initiation and propagation are captured using a phase-field approach, enabling a mesh-insensitive description of fracture evolution. The effects of boundary conditions and heating rates are systematically examined. Numerical results demonstrate that boundary conditions significantly influence the failure mode and damage evolution of concrete specimens. The constraint location determines crack initiation sites and propagation paths, while the constraint intensity governs crack distribution characteristics. In addition, the number of heated boundaries alters the temperature field, leading to distinct damage localization patterns. With increasing heating rate, the onset of failure is markedly advanced, and the spalling mode exhibits a clear transition from deep, through-thickness fracture in the specimen core to localized corner spalling near exposed surfaces. The predicted damage evolution and fracture patterns show good agreement with experimental observations reported in the literature[1], highlighting the competing effects of thermal expansion and pore pressure in driving fracture under fire exposure. The proposed multiphysical phase-field framework provides a robust computational tool for elucidating the mechanisms of high-temperature spalling in porous concrete materials and offers valuable insights into the regulation of damage patterns through boundary condition design and thermal loading control.