A control device and a method for controlling scanning speed of a scanner. The control device includes a decision device and a driving device. The decision device further includes an image buffer, an up-down counter and a comparator. The decision device receives the input image data and utilizes the up-down counter to compute data access volume inside the image buffer. The comparator decides whether to increase or decrease the scanning speed according to the data access volume and also outputs decision data to the driving device.