A scaling-up control method and a scaling-up control apparatus are used in a display device. The display device includes a scaler for converting an original image signal into an output image signal by performing a linear interpolation. Moreover, by utilizing a global locking mechanism and a local locking mechanism to perform the linear interpolation to determine the actual position of the interpolated pixel, the position error of the pixel position is largely reduced. Since the difference between the calculated pixel value and the ideal pixel value is reduced, the imaging quality of the output image signal is enhanced.