Transcranial photobiomodulation (tPBM) is a non-invasive neuromodulation technique utilizing infrared or near-infrared light. Clarifying the energy deposition patterns and safe dose thresholds within realistic anatomical head structures is crucial for advancing its clinical application. However, existing studies often rely on simplified models for numerical simulation, which may fail to accurately capture the influence of cortical sulci and gyri on light propagation and thermal diffusion. This study established a high-resolution (1 mm3) anatomically realistic human head model using the finite element method, and developed an optical-thermal multiphysics coupling framework to systematically simulate and analyze key parameters, including power density and wavelength, to evaluate light penetration depth and tissue temperature elevation characteristics. The results demonstrated that approximately 0.05% of the incident optical power penetrated to the cortical gray matter. As the power density increased, scalp temperature elevation reached up to 5.22 °C, and brain temperature elevation reached up to 0.49 °C, with a distinct “halo” scattering effect observed within the cerebrospinal fluid layer. These simulation results systematically revealed the photothermal propagation characteristics and tissue thermal responses of tPBM in realistic head anatomy, providing an important theoretical basis for defining safe dose thresholds and optimizing individualized stimulation parameters.