1. An automatic detection method for defects of a display panel, comprising: acquiring a tag image, a mapped original image and a mapped tag image; dividing the mapped original image into a plurality of mapped original sub-images, and dividing the mapped tag image into a plurality of mapped tag sub-images; acquiring a normal area and a defective area of the mapped original sub-images; merging the mapped original sub-images to discriminate the normal area and the defective area of the mapped original sub-images; correcting the discriminated normal area and the discriminated defective area of the mapped original sub-images by using the mapped tag image and the tag image to acquire a defect location of the display panel; wherein the step of acquiring a tag image, a mapping original image and a mapping tag image further comprises: marking a plurality of tag points in the original image displayed on the display panel to acquire the tag image; sampling the original image and the tag image to acquire a sampled original image and a sampled tag image; respectively cutting the original sampled image and the tag sampled image by using a plurality of corner points, to acquire a cut original image and a cut tag image; respectively rotating the cut original image and the cut tag image to acquire a rotated original image and a rotated tag image; respectively cutting again and stretching the rotated original image and the rotated tag image to acquire a stretched original image and a stretched tag image; respectively black-pixel-inserting the stretched original image and the stretched tag image, and filling a black pixel by using a linear interpolation method to acquire the mapped original image and the mapped tag image.