Abstract: The paper deduced formula of electromagnetic wave attenuation caused by wall roughness under combined effects of multi wave mode according to transmission theory of multi wave mode, and simulated and analyzed effect of wall roughness on attenuation characteristics of electromagnetic wave by multi mode model. Matlab simulation results show that influence of wall roughness on attenuation characteristic increases with frequency. Due to combined effects of waveguide and wall roughness, there is a frequency range with low attenuation of 0.4-2 GHz in wave propagation in coal mine tunnels.