A warning system applied to a vehicle includes a control module, a detecting module, a warning module, and a pressure sensor module. The pressure sensor module is assembled to a seat, and configured to measure a pressure value of the seat and transmit the pressure value to the control module. The detecting module is configured to obtain information of the environment around the vehicle and transmit the information to the control module. The control module is configured to determine whether there is at least one person sitting on the seat based on the pressure value on the seat, and to determine whether there is a passing vehicle based on the information of the environment surrounding the vehicle, and warn the passing vehicle when there is someone sitting on the seat and there is a passing vehicle.