A monitoring system includes a controller, a number of input devices, a number of first sensors, a number of second sensors, and a number of alarms. Each input device comprises a switching circuit, an input circuit, and a connector. The connector is connected to a first sensor and a voltage source in series or connected to a second sensor and a current source in series. The controller controls the switching circuit of the input device to receive a voltage signal of the first sensor or a current signal of the second sensor and send out a corresponding detection signal. The input circuit transmits the detection signal to the controller. The controller controls the corresponding alarm to work according to the detection signal.