A device and a method for detecting a passenger within a vehicle are provided. The device includes a sensor mounted inside the vehicle and a vehicle network connection device that receives a position and a backrest angle of each seat in the vehicle and transmits a control signal for adjusting the position and the backrest angle of each seat in the vehicle. A controller performs primary detection using the sensor in a precise detection mode determined based on the received position and backrest angle of each seat in the vehicle, and performs final detection using the sensor in a general detection mode while adjusting the position and backrest angle of each seat in the vehicle.