Large-core fibers are wildly used in astronomical applications. For multi-object fiber spectroscopic telescopes, for example the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), the aiming accuracy is important. The fibers in these telescopes have two ends; one input end is in the focal plane to collect the star image while another output end is inside a spectrometer. If the input end doesn't exactly match the image of the star, the emitting spot from the output end could not be a central-peak circular spot. If the misalignment is serious, the output would be a ring. The ring spot would lead to wrong spectrum analysis result. To obtain the relationship of the input position and the output spot, we designed a scanning experiment. We chose a single-mode fiber for 650nm, which core diameter is 4 μm, to scan a large-core astronomical fiber FBP320385415, which is used in LAMOST. The experimental results show that if the input point is 122μm away the center of the fiber, the output spot will be a ring spot instead of the supposed circular pattern. The nearer of the input position is to the edge of fiber, the closer the output spot to a ring. This experimental result is important for designing and optimizing the fiber-end adjusting devices of multi-object fiber spectroscopic telescopes. It can also apply for various fields of astronomical spectroscopy observation.