An AC input voltage detecting device includes a first input, a second input, a determining unit, a first voltage-detecting unit, and a second voltage-detecting unit. The first input and the second input are electrically connected to a neutral line and a live line of an AC power, respectively. The determining unit includes a first power switch, a second power switch, and an output, the first and second power switch are electrically connected in series, and the output is coupled to the second power switch. The first voltage-detecting unit is electrically connected to the first input and the first power switch, the second voltage-detecting unit is electrically connected to the second input and the second power switch. A signal for indicating that the AC power supplies normally is generated from the determining unit while the voltages conducted by the live line and the neutral line are normal.