For analyzing the influence of emitted electrons velocity and direction distribution on the performance of terahertz Folded Waveguide Traveling Wave Tube(FWTWT), the kinetic and angular spread modules of a massively parallel 3-D fully electromagnetic and Particle In Cell(PIC) code named NEPTUNE3D are designed. The above modules are applied to the simulation of 0.22 THz FWTWT successfully. The simulated results could be concluded as followings. The kinetic spread alters the range of electron-beam voltage distribution which destroys the oscillation voltage relation between electron beam and electromagnetic wave, therefore causes the degrading of device performance. The angular spread reflects the distribution of emission angle which alters electron transverse and axial velocities. As the increasing of transverse velocity, many electrons will collide with the inner-wall of tube which causes the degrading of device performance. At the same time, considering the constant beam voltage, the decreasing of electron axial velocity could destroy the synchronous condition between beam and wave, which causes the device performance degrading furthermore.