An alarm system for emitting an alarm includes a distance detecting circuit, a gate circuit, a voltage conversion circuit, a distance judging circuit and an alarm circuit. The distance detecting circuit receives a first signal, detects a distance between an object and the alarm system, outputs a second signal and a third signal according to the distance. The gate circuit receives the second signal and the third signal and outputs a voltage level signal. The voltage conversion circuit receives the voltage level signal and converts the voltage level signal into a DC voltage signal. The distance judging circuit receives the DC voltage signal and compares the DC voltage signal against a reference voltage signal to output a control signal if necessary. The alarm circuit receives the control signal and produces the alarm when the distance between the object and the alarm system is less than a threshold distance.