A monitoring system includes a controller, a number of sensors, a number of alarms, and a number of input/output devices. Each input/output device is connected between the controller and a sensor or an alarm. Each input/output device includes an input circuit, an output circuit, and a connector to selectively connect to the sensor or the alarm. The input circuit and the output circuit are connected between the controller and the connector. The controller controls status of the input circuit and the output circuit. When the connector is connected to the alarm, the controller controls the alarm to work via the output circuit. When the connector is connected to the sensor, the sensor outputs a detection signal to the controller via the input circuit.