The equivalent-conductivity method is applied to the simulation work of electromagnetic transmission attenuation for 310 GHz double vane slow-wave structure. The factors which will influence the transmission attenuation，including surface roughness and harmonic mode, are studied. The result shows that roughness will result in serious electromagnetic attenuation. The -1st and the 1st harmonics at the same frequency result in different transmission attenuations; and the -1st harmonic results in greater electromagnetic attenuation which will increase faster when roughness grows up. The process of the beam-wave interaction of structure is also simulated. Especially the influence of roughness on working performance of Strategic Weapons Systems(SWS)，such as gain, bandwidth，is analyzed.The results show that roughness causes gain reduction and bandwidth decrease.