The observation and analysis of microstructure of crop stalk are of great value. Based on the structure and gray features of tissues in wheat stalk micrograph, a sector ring region-based image segmentation method was proposed, by which the sclerenchyma and vascular bundles in the micrograph would be separated from the background precisely. The inner and outer contours of the wheat stalks cross-section were got from an approximate annular. The annular that the sclerenchyma and vascular bundle belonged to was firstly located roughly. On the basis of the location information, the center of the section was taken as the origin and drawn rays towards to the outer contours of the annular. By this operation, the annular was divided into many small sector ring regions. Then small regions were segmented respectively by using the Otsu algorithm. Finally, the segmentation results were processed such as superposition, and the complete sclerenchyma or vascular bundle would be obtained. Using the segmentation results all kinds of parameters of the section would be evaluated accurately. It was showed that the method presented had higher accuracy and stronger robustness than the traditional Otsu method in segmenting the low-quality image which caused by slicing, dying and photographing in the process of micrograph acquisition. It improved the utilization rate of the slice sample and reduced the labor intensity. The accuracy of all the parameters obtained by using the measurement system was more than 94.6%, which would play an important practical role in the research of wheat stalk microstructure.