Aerodynamic heating and heat sink scarcity pose critical challenges for the Environmental Control System （ECS） and thermal management systems of supersonic civil aircrafts. Current research on ECS primarily focuses on subsonic aircraft， with insufficient attention to supersonic configurations， necessitating dedicated investigations into supersonic civil aircraft ECS. Considering the available heat sinks on the aircraft during supersonic flight， a ECS integrated into the fuel thermal management system with heat sink mode switching function is proposed. Thermodynamic modeling and simulation were conducted for the systems. Simulation analysis under typical flight mission profiles demonstrated that this integrated system satisfies critical thermal safety boundary conditions， including maintai-ning cabin supply air temperature below 18 °C and fuel temperature under 150 °C during supersonic cruise and dece-leration descent phases. The research reveals that when the design endurance increases during cruise phase or initial fuel temperature rises， relying solely on fuel heat sinks may result in system thermal endurance falling below design requirements. Switching to ram air heat sink mode in such scenarios can extend thermal endurance. During deceleration descent， simultaneous utilization of both fuel and ram air heat sinks becomes necessary， where thermal endurance becomes co-constrained by fuel temperature limits and supply air temperature requirements. Regulating the recirculating fuel flow rate and ram air flow rate can further extend thermal endurance under these dual constraints. The dual-heat sink mode switching strategy balances thermal safety and economy， providing theoretical support and enginee-ring optimization pathways for the design of integrated thermal management systems in supersonic civil aircraft.