Abstract: Based on a large-scale parallel code named NEPTUNE3D for 3D fully electromagnetic and PIC simulations programmed by our group, we design and develop a Finite Conductivity Wall (FCW) module in order to simulate the gain loss phenomenon in THz traveling-wave tube caused by metal conductivity and roughness. First, the advantage and disadvantage of FDTD scheme in metal material and harmonious conditions are introduced. Then, we put forward an implicit scheme of FCW and discuss its numerical flow. This method is of unconditional stability and good expansibility. Moreover, we test the FCW module with an example of electromagnetic wave mode transmission in a rectangular wave guide. Compared with theoretical results and commercial electromagnetic software simulation results, the FCW module is validated. By using the improved NEPTUNE3D code after adding FCW module, we simulate an 0.22 THz folded-waveguide traveling-wave tube (FWTWT) gain loss course caused by copper surface roughness. The simulation results indicate that metal surface roughness causes a serious gain loss effect on FWTWT device. Finally, the dispersion relation, interaction impedance and attenuation of device are analyzed, some structure design advices are presented. For increasing the output power, increasing signal power, beam current and structure period number is feasible in some ways.