In order to improve the flatness and adjustability of spectral characteristic of multi-channel absorber,an electromagnetic metamaterial multi-channel absorber with five-layer structure is designed based on Si substrate/aluminum base/silver single slit /graphene /silicon dioxide dielectric.The multi-channel absorber not only has easily tuned absorption spectrum characteristic,but also improve the flatness of each channel by 2.84 dB.The influence rule of structural design parameters on the absorption spectrum is theoretically analyzed based on the finite-difference time-domain (FDTD),and the optimal structure is obtained.Simulation results show that the flatness of absorption channel can be improved by both the SiO₂ dielectric layer on the top of absorber and the single slit filled with Au,while the number of absorption spectral channels,channel spacing and single-channel bandwidth can be effectively tuned by adjusting the width of the top layer with SiO₂ or the upper layer with graphene.Especially,the absorption frequency band and absorptivity of the absorber can be selected by changing the Fermi energy level of the graphene.The absorber has potential applications in many fields such as biochemical testing,environmental monitoring and smart sensing.