Abstract: In this paper, the normal single-mode fiber is pulled into microfiber by the method of carbon dioxide laser heating, wet method is used to transfer graphene and cover microfiber to form a composite waveguide. The light of different wavelengths enters the composite waveguide through the coupler, interacts with graphene in the form of evanescent waves, thus the research experiment of preferential absorption characteristic is carried out. When a short wave light was used as a pump light, changes in the long wave spectrum with the increase of incident intensity were measured, obtaining a modulation depth of about 3.5 dB, and a modulation efficiency of 0.62 dB·mW^-1. When the long wave light was used as the pump light, the light transmittance of the short wave signal was measured to change by ~1.9%. The experimental results show that with the increase of the incident light intensity of the pump light at any wavelength, the composite waveguide exhibits its preferential absorption characteristics. In addition, when the long-wave and short-wave waves passed through the composite waveguide, the transmittance changes with the input power were tested, experiments show that the long-wavelength transmittance increases faster than the short-wavelength transmittance, and the corresponding theoretical analysis can be made from the two aspects of energy band and evanescent wave.