Abstract: Based on the combination of Finite-Difference Time-Domain and Particle-In-Cell method, a 3D PIC simulationprogram is developed to study the nonlinear beam-wave interaction for helix TWT. By calculating the Maxwell Equations andLorenz Equation on high-speed computer, a C-band helix TWT is simulated. From the simulation results we can obtain theparameters of the tube including output power, gain, electronic phase, etc.