Cotton ginning quality is very important to evaluate the cotton grade. Cotton defects influence cotton and textiles qualities. It is very significant to measure cotton defects rapidly. Cotton defects include seed coat fragment, bearded motes, and ginned dead cotton which have different light response characteristics. A novel method is proposed to measure the cotton defects based on optoelectronic measurement technique. The image segmentation is made by using adaptive thresholds and cotton defects can be inspected correctly. The best segmentation threshold of seed coat fragment and bearded motes is 0.6. The dead cotton′ s best segmentation threshold is 0.5. The comparison experimental results show that the inspection accuracy of measuring system is more than 85% and the measuring time is less than 3 seconds. The measuring system can meet requirement of cotton defects rapid inspection.