The present invention provides a voltage control method for controlling a power supply. The voltage control method comprises the following steps: obtaining a present output voltage value associated with a present gain value; obtaining a predetermined output voltage value associated with a predetermined duty ratio; calculating a target gain value, corresponding to the predetermined duty ratio, according to a gain value formula; performing a weight calculation on the present gain value and the target gain value for generating a buffer gain value; and setting an output voltage command according to the buffer gain value. Wherein the buffer gain value is between the present gain value and the target gain value.