The invention discloses a tobacco length and width distribution automatic detection method and detection system. The detection method comprises the following steps that: 1) tobacco is separated and is made to suspend; 2) the original image data of the tobacco are acquired; 3) point cloud data are generated; 4) a tobacco three-dimensional model is restored; 5) aggregation and segmentation are performed, and the curvature of each individual object is calculated, key points are determined according to a certain change ratio, and the three-dimensional distances of the key points are calculated, and length and width data are obtained, and the length and width distribution of the tobacco is put into statistics; and 6) the length and width distribution graph of the tobacco is outputted. The detection system comprises a top cover, a tobacco separation cylinder, an air volume regulating cylinder, an air feed duct and a fan; the detection system further comprises an air compressor, a numerical control box and a computer; and a three-dimensional scanning sensor and an air nozzle are arranged in the tobacco separation cylinder. With the tobacco length and width distribution automatic detection method and detection system of the invention adopted, accurate measurement of the length and width distribution of the tobacco can be realized, and an accurate and rapid detection system is provided for the analysis of the structure of the tobacco.