Abstract: To reveal the evolution law of the temperature field inside submarine tunnels under the coupled effects of external environmental temperature variations and construction cycles, and to optimize ventilation parameters, both field monitoring and numerical simulations were conducted. First, the non-steady-state evolution characteristics of the tunnel temperature field were analyzed based on field measurements. Then, a three-dimensional ventilation-heat transfer numerical model considering cutterhead frictional heat generation was established to comparatively investigate the dynamic thermal responses of the tunnel during excavation and shutdown conditions. Adaptive optimization of ventilation parameters was subsequently performed for representative construction stages. The results show that: (1) The temperature distribution along the tunnel is not affected by construction stoppages. When the tunnel advances to 6,932 m and the ambient temperature is 24.3 °C, the temperature from the first segment car to the 1,300th ring exhibits a “rapid drop (cutterhead-ring 3,350) followed by a gradual decline (ring 3,350-ring 1,300)” pattern, and becomes stable beyond 3 km behind the tunnel face. (2) During excavation, the airflow around the first segment car forms multiple vortices, leading to heat retention and a relatively higher temperature; after stoppage, the temperature initially rises slowly and then gradually decreases. (3) External temperature fluctuations exert significant influence on the tunnel′s internal temperature, and ventilation airflow should be dynamically adjusted according to different construction phases.