A method of controlling a parking control device of a transmission may include a) inputting data about a state of a vehicle, b) determining, based on the data input at the step a), whether a current situation is a first situation in which an actuator of the parking control device of the transmission is to be promptly controlled, a second situation in which noise is controlled to be minimized, or a third situation which is a situation other than the first and second situations, and c) controlling a Pulse Width Modulation (PWM) duty cycle required to control the actuator to different values depending on the first, second, and third situations determined at the step b), thus driving the actuator.