The key factors of nonlinear fiber devices are analyzed from the loss aspect. The dependences of insertion loss, reflection loss and intrinsic loss on the input optical power are studied. In the case of high input optical power, the insertion loss is proportional to the input optical power due to the stimulated Brillouin scattering (SBS), and the slope is related to the SBS efficiency. The changes of loss are measured in fiber spools with different diameters. The SBS threshold and the efficiency of four-wave mixing (FWM) in the fiber spool with small diameter are also investigated experimentally. The results indicate that the nonlinear performance of optical devices with a 7cm-diameter fiber spool is basically the same as that of the fiber spool with the conventional diameter, which helps to design the toroidal coil with the magnetic field along the fibers and analyze the characters of the nonlinear magneto-optic fiber.