Considering the attenuation, severing, taper and magnetic admittance, the self-consistent working equations are presented, which describe the operation of 3D nonlinear beam-wave interaction for helix traveling wave tube based on periodic structure characteristic and Poynting's theorem. Using Fourier transform from time-dependent current to ac current consisting of a series of wave frequencies, the space fields are obtained through weighing charge particles to fixed meshes with the help of particle_in_cell technique. The model also computes 3D electronic tracks and helix interception current, the electronic energy spectrum of spent beam are represented, which is helpful for designing multistage depressed collector （MDC）. Calculation of Ku-band TWT in IECAS was shown to be in better agreement with experiment, including output power and analysis of MDC.