The invention discloses a tobacco stem expansion rate measuring system and method based on image processing. The system comprises a support, a camera, a power device and an image processing device. The power device comprises a motor and a vertical rotating shaft. A carrying disc is fixedly arranged at the upper end of the rotating shaft. The motor is fixedly arranged on a fixed based on the support. The camera and the carrying disc are arranged at the same horizontal height. The signal output end of the camera is connected with the signal input end of the image processing device. The method includes the following steps of firstly, preparing to-be-measured tobacco stems; secondly, measuring the sectional areas of the to-be-measured tobacco stems; thirdly, calculating the average value of the sectional areas of the tobacco stems; fourthly, conducting expansion processing on the to-be-measured tobacco stems, measuring the sectional areas of the to-be-measured tobacco stems subjected to expansion processing, and calculating the average value of the sectional areas; fifthly, calculating the expansion rate of the tobacco stems. The system and method have the advantages of being easy to operate, low in detection cost, high in efficiency and high in detection result accuracy.