Abstract: During the operation of EMU trains, the air conditioning system relies on air supply, air return and exhaust to collaboratively realize the environmental control of the passenger compartment and the supply of fresh air. However, under sudden changes in external pressure, airflow backflow may occur in the exhaust system. The backflow can carry external dust into the PIS cabinet and cause false alarms from the smoke sensors. In this study, based on full-scale train tests, pressure and dust concentration sensors are installed inside and outside the exhaust outlet of the PIS cabinet, and reflective ribbons are placed inside the cabinet and observed by video surveillance to reveal the triggering mechanism of airflow backflow in the exhaust duct. The results of the study show that the aerodynamic effects of tunnels or train passing cause the pressure at the outlet of the vehicle exhaust duct to rise, which triggers airflow backflow when the exhaust damper is closed at an inappropriate time. When the concentration of solid pollutant particles in the external atmosphere is high, the backflow will significantly increase the dust concentration in the PIS cabinet, triggering false alarms from the smoke sensors; during actual line operation, the dust backflow phenomenon due to the delayed closure of the dampers will lead to a maximum increase in the dust concentration in the PIS cabinet by 12.5%.