The possibility of applying the focusing method to measure the refractive index profile of polymer optical fibers is studied through combination of the theoretical analysis and experimental verification. The result shows that the focusing method is an easy and feasible way to determine the refractive index of graded-index POFs; however, as the light rays cross each other near the POF boundary, the focusing method is not an appropriate way to decide accurately the refractive index profile of step-index POFs.