Ray tracing was used to analyze the Focal Ratio Degradation (FRD) properties of fibers, which is caused by the light transmitting in the fibers with offset and bending when we set the fibers and observe stars in astronomy. We analyzed and calculated the conditions of the fibers used in the Large Sky Area Multi-Object Fiber Spectroscopy Telescope(LAMOST) according to the parameters of the fiber cables used in the LAMOST. These works provide useful reference in this project. According to the analysis and computation, the designed aim error of fibers should be less than 32 μm at least.The least bending radii is 300 mm when the fibers are installed for observation in the LAMOST projection.