1. A face-slimming method applied to a series of images captured by an electronic device, comprising: receiving a current image; detecting at least a face landmark corresponding to a face region of the current image; comparing the face landmark of the current image with a reference landmark; adjusting a distortion mask according to the comparison result; and adjusting a size of the face region within the current image according to the distortion mask, wherein the distortion mask corresponds to a position of the face landmark, wherein the comparing of the face landmark comprises: determining whether the position of the face landmark falls within a stable window; and increasing an unstable count number in response to the position of the face landmark falling outside of the stable window, where the stable window is defined according to the position of the reference landmark, wherein the comparing of the face landmark further comprises: determining whether the unstable count number exceeds a predetermined threshold; and updating the reference landmark with the face landmark of the current image in response to the unstable count number exceeding the predetermined threshold.