<p>Thermal huff-and-puff production using horizontal wells offers a promising strategy to overcome the limitations of depressurization in low-permeability marine gas hydrate reservoirs, where slow pressure propagation and insufficient heat supply constrain gas recovery. However, its productivity and geomechanical response are governed by strongly coupled thermal–hydraulic–mechanical–chemical processes, the underlying mechanisms of which remain poorly constrained. Here, based on reservoir parameters from the Shenhu area of the South China Sea, we develop a three-dimensional, two-way coupled THMC model and validate it against data from the second field production test. The model is then used to systematically investigate the effects of soaking time and reservoir permeability on production performance and multiphysics responses. The results show that neglecting geomechanical feedback substantially overestimates reservoir flow capacity and gas production potential. An optimal soaking time exists at approximately 20 days: moderate soaking improves heat-use efficiency and promotes hydrate dissociation, whereas excessive soaking intensifies heat loss into surrounding formations and the overburden, thereby weakening the stimulation effect. Increasing reservoir permeability markedly enlarges the mobilized reservoir volume and enhances gas production, but also amplifies the magnitude and spatial extent of formation subsidence. Overall, the coordinated optimization of moderate soaking and reservoir stimulation is critical for balancing productivity enhancement with geomechanical safety. This study provides a theoretical basis for understanding THMC coupling mechanisms and optimizing operational parameters in thermal huff-and-puff production of marine gas hydrates.</p>