A passenger occupancy detection system is provided that implements a sensor-connected passenger detection method. The system includes that a sensor controller that reads a sensor value using an ESC as an acceleration sensor when a vehicle is stopped and distinguishes an in-vehicle passenger with a vehicle acceleration change by the sensor value. The operations of a rear-seat passenger notification system 200 and a passenger-seat occupant classification advanced airbag system 300 are connected by the distinguishing the in-vehicle passenger, enhancing the sensor reliability by providing Fail-Safe together with increasing the accuracy of the passenger detection utilizing longitudinal/lateral accelerations/yaw rate information.