<p>Soil thermal properties regulate temperature dynamics and heat transfer, thereby affecting crop growth, microbial activity, water evaporation, and the thermal stability of subgrade and foundation soils. However, the combined effects of biochar, salinity, and soil moisture under high compaction remain insufficiently studied. This study evaluated the influence of biochar (0%, 5%, 10%) and salinity (0 and 2% NaCl) on the hydro-thermal and water retention behavior of highly compacted (95%) granite residual soils (GRS) through laboratory column tests. Results show that biochar enhances soil water retention by increasing porosity and hydrophilicity, with stronger effects at 10%, while also altering the physicochemical and thermal properties of GRS and salt-affected soils (SGRS). Incorporation of 10% biochar reduced thermal conductivity by 11–31%, volumetric heat capacity by 2–48%, and thermal diffusivity by 1–14%, while increasing thermal resistivity by 12–45%. Salinity exerted a stronger effect at low saturation, whereas at high saturation (S_r = 0.9), SGRS exhibited higher thermal conductivity and heat capacity than GRS, highlighting the coupled influence of water and salt on soil thermal behavior. Moreover, soil water content exerted greater control on thermal properties than biochar addition. Model evaluation showed that the Campbell model (RMSE = 0.14–0.32), Zhao et al. model (RMSE = 0.33–0.38), and modified Zhao et al. model (RMSE = 0.07–0.23) accurately predicted thermal parameters. These findings support biochar use to regulate soil thermal and hydraulic behavior, improving soil conditions, water use, and temperature control in saline-alkaline farmland, and enhancing thermal stability and deformation resistance of subgrades and foundations.</p> Graphic Abstract <p></p>