A kind of radar-absorbing material (RAM) based on metamaterial, which contains resistive patches, dielectric substrate and metal backing, is designed. The reflectivity of the RAM is calculated using the finite difference time-domain method (FDTD) and optimized by genetic algorithm(GA). Then RAMs with a reflectivity bandwidth of 818 GHz below -10 dB is obtained. Moreover, its thickness is only 3mm. To confirm the design results, the RAM is prepared by fabricating a hollow quartz fibre reinforced epoxy resin substrate through using hand lay-up process and making the resistive patches by screen painting. The reflectivity of the absorber is measured, showing that the experimental result is consistent with the design value.