Traditional Tujia wooden dwellings are important carriers of regional architectural culture, yet research on their indoor thermal environment and climate-adaptive optimization remains limited. This study investigates a typical “‘一’-shaped, three-bay” Tujia traditional timber dwelling in northwestern Hunan, China, using a combination of field surveys, on-site measurements, occupants’ thermal sensation questionnaires, and numerical simulation to quantitatively assess its summer indoor thermal environment and envelope optimization strategies. The results show that these dwellings generally exhibit hot and humid indoor conditions in summer, with marked thermal differences among functional spaces. The fireplace room demonstrates the poorest thermal performance, while the bedrooms perform relatively better. Simulation calibrated against field measurements further indicates that, without substantially altering the traditional architectural character, indoor thermal comfort can be effectively improved by enhancing the thermal performance of the envelope and optimizing openings, particularly window design and the window-to-wall ratio. The findings suggest that traditional Tujia wooden dwellings in northwestern Hunan embody valuable climate-responsive construction wisdom and retain significant potential for further improvement under contemporary living requirements. This study can provide a reference for the sustainable conservation, adaptive renewal, and improvement of rural living environments of traditional timber dwellings in northwestern Hunan and other regions with similar climatic conditions.