Plastic optical fiber(POF) is widely used in the detection of high-energy physical cosmic rays. The quality of the optical fiber end face determines the performance of the detector largely, thus affecting the accuracy of cosmic ray spectrum and composition measurement. In this paper, the BCF-92 POF used in the electromagnetic particle detector of the high altitude cosmic ray project is taken as the research object. The image of the optical fiber end face is preprocessed by the way of Matlab image processing. The geometric parameters of the end face of the core cladding are determined by the iterative least square ellipse fitting method, and the end face out of roundness is calculated. The experimental results show that 95.4% of the sample optical fiber is qualified, and the average non circularity of the end face is about 0.75%. This method can meet the detection requirements of high altitude cosmic ray project.