During the lens automatic detection ba sed on machine vision technology,the defects should be classified in order to ac hieve the grading of the lenses.Automatic detection system components, working principle and the resin lens major defects are simply introduced.As for the classification of the three main defects of point impurities,scratches and feathers,the circularity and straight-line fitting correlation have been propos ed in this paper as the classifying features.The threshold of the classifying features has been calculated on the basis of data analysis and statistics.The i nfluence of the thinning algorithm on the straight-line fitting correlation has been discussed for the defects of scratches and feathers.The experimental results show that the thinning algorithm plays an imp ortant role in improving classification accuracy of scratches and feathers.The experiment is conducted to analyze the feasibility of the algorithm.The results indicate that this method has a high c lassification accuracy of 96% for the three defects,which meets the requiremen ts of the detection system.