To solve the problem of low measurement efficiency due to the lack of octagonal optical fibers in traditional fiber end face size detection methods, image processing is used to design the algorithm for octagonal fiber size detection. For the three parameters to be measured, namely, fiber core diameter, cladding edge to edge distance, cladding diagonal distance, Gaussian filtering is used to denoise the image, and subpixel contours in different areas of the fiber to be measured are extracted. The cladding contour is divided into Octagon fiber contours using the Ramer algorithm, Propose a segmentation processing algorithm to process multi segment contour edges of different lengths and obtain effective eight segment contours; Use the least squares method for curve fitting, measure the diameter of the fiber core contour as a circle, and measure the distance between the edges of the cladding contour as a straight line; A vertex detection algorithm based on auxiliary line is proposed to measure the diagonal distance. Experimental results show that this algorithm can accurately and quickly measure the core diameter, four groups of edge to edge distance and four groups of diagonal distance of octagonal fiber,The average accuracy of repeated measurements of opposite edges and diagonal distances reaches 0.1 μm and meet the requirements of enterprise technical indicators.