A voltage-modulated circuit device uses a power source of a modulation circuit to acquire a predetermined trigger voltage through a voltage acquisition circuit and a step-down circuit to step down a voltage, and then a voltage amplification circuit amplifies the voltage by several times, such that the trigger voltage amplified by several times still falling within the range of predetermined voltage values triggers and conducts a switch circuit, and the power source of the modulation circuit can flow from an anode to a cathode in order to supply an electric power with a step-down voltage. With the design of triggering the conduction, the voltage of the modulation circuit can be modulated automatically to maintain the electric power supplied to the circuit device within the range of required voltage values, only if the trigger voltage has the predetermined voltage value.