Urban underground space is an important natural resource, and making full use of this space is crucial for urban sustainable development. Vigorously developing underground cable tunnels is an effective strategy to meet the growing demand for power supply in cities. However, the thermal and humid environment in cable tunnels significantly impacts their operation, with condensation posing a major threat to electronic equipment and cables. To address this, a long-term field monitoring method was used to track the temperature and humidity of a cable tunnel in Shanghai for 10 months. The heat transfer mechanism within the tunnel was analyzed, and the temperature and humidity distribution within the tunnel body and the ventilation shaft equipment room were studied. The influence of the external environment on the internal conditions of the cable tunnel was also examined. The results show that temperature and humidity distribution in the cable tunnel is not uniform along its axial direction. Due to natural ventilation, the temperature near the air supply shaft is highly variable, while the temperature near the exhaust shaft is relatively stable at about 25.0°C. Influenced by external environmental changes and heat accumulation from the cables, both the air temperature and the wall temperature in the tunnel increase gradually, with the temperature variation range decreasing from the air supply shaft to the exhaust shaft. From June to August, the daytime outdoor air dew point temperature in Shanghai can reach 25°C. Since the wall temperature at the tunnel entrance is lower than the outdoor air dew point temperature, condensation occurs when outdoor air enters the tunnel. Therefore, air dehumidification should be implemented at the air supply shaft. Additionally, this paper explores methods for controlling humidity in cable tunnels. During transitional seasons, mechanical ventilation and dehumidification using fans can be employed, but special attention is needed for specific weather conditions.