1. An optical film defect detection method, comprising the following steps: scanning a brightness value of each of lines of an optical film by an optical detector, and calculating a brightness average value of each line, obtaining a brightness weight value corresponding to each line of the optical film and multiplying a plurality of pixels of the optical film and the brightness weight value corresponding to the plurality of pixels of the optical film to form a brightness uniformity image by a processor; detecting the brightness uniformity image by the optical detector to obtain a grey scale value of the plurality of pixels, counting a frequency of occurrence of the grey scale value by the processor, and converting the grey scale value into a grey scale level value corresponding to the grey scale value by a grey scale histogram equalization, so as to form a reinforcement contrast image; finding out the grey scale level value in a peripheral region of the plurality of pixels in the reinforcement contrast image by the processor and replacing the grey scale level value of the plurality of pixels by a morphological opening operation or a morphological closing operation, so as to form a noise filtering image; finding out the grey scale level value in a peripheral region of the plurality of pixels in the noise filtering image by the processor and replacing the grey scale level value of the plurality of pixels by an intermediate value of a sequenced grey scale level value, so as to form a smooth image; comparing the grey scale level value of the plurality of pixels in the smooth image with a threshold value by the processor and dividing the plurality of pixels of the smooth image into a black defective part and a white defect-free part by a binary conversion, so as to form a clean detection image; counting a relative relation between the plurality of pixels and adjacent pixels having a spacing distance in the clean detection image and calculating a characteristic value of the relative relation in different spacing distances by the processor, so as to form a spatial relation sequence model having the spacing distance and the characteristic value, and comparing the spatial relation sequence model with a defect sequence model stored in a memory by the processor to determine a defect type of the optical film.