1. An image processing method of improving human face recognition, applied to a human face recognition system, the image processing method comprising: capturing an image including a human face; executing a face detection in a predetermined area of the image, wherein the predetermined area is smaller than the image, and the predetermined area is in a central area of the image; adjusting an exposure value of the predetermined area to an expected value when the face detection does not detect a face area of the human face in the predetermined area of the image; when the exposure value of the predetermined area reaches the expected value, executing the face detection to detect the face area, and analyzing an image information in the face area; selecting a model parameter and an image adjusting parameter corresponding to the model parameter according to the image information and processing the face area of the image, so as to improve a clarity of dark areas of the face area in the image after the face area is detected by the face detection; and outputting a processed image to the human face recognition system; wherein the image information includes a luminance histogram and an average brightness of the face area; wherein the image adjusting parameter is a wide dynamic range (WDR) parameter; wherein the step of processing the image includes: executing a WDR processing on the image based on the WDR parameter; and wherein in a circumstance of back lighting or low ambient light, the average brightness of a central area is increased by gradually adjusting a threshold according to a lookup table.