Provided is a device and a method for adjusting a loop filter gain in an automatic frequency controller, which can allow the automatic frequency controller to operate while maintaining an optimal performance. The device includes a velocity estimator for estimating a change in velocity in accordance with a movement of a terminal; a loop filter gain controller for controlling a loop filter gain in accordance with a change in velocity received from the velocity estimator; and a loop filter gain unit for multiplying a loop filter gain received from the loop filter gain controller and a frequency error for output. Further, there is provided a method for adjusting a loop filter gain in an automatic frequency controller, having the steps of: estimating a change in velocity in accordance with a movement of a terminal; adjusting a loop filter gain in accordance with the estimated change in velocity; and multiplying the adjusted loop filter value and a frequency error for output.