1. A processing method for image interpolation, comprising: detecting whether an edge exists for a selected area in an original image by using an edge detection method, and determining an edge direction of the edge; on a normal direction to the edge direction, producing a contour by using a plurality of pixels in the original image to interpolate a plurality of contour pixels, wherein a destination pixel is located on the contour; identifying two extreme points from the contour pixels by judging a changing trend of a brightness value of the contour; determining whether the destination pixel is located at a transition area or a non-transition area; when the destination pixel is located at the non-transition area, calculating an interpolation adjust value according to the brightness values of the two extreme points and the brightness value of the destination pixel; and obtaining an interpolation result of the destination pixel by performing the interpolation on the selected area according to the interpolation adjust value.