Abstract: The accurate three dimensional helix travlingwave tube coldtest simulation model is presented using the electromagnetic code HFSS. For electric field in the perturbation rod, four prominent approximations of perturbation method are analyzed with the simulation model. Firstly, the electric field is supposed uniform in the axial coordinate, neglecting the higher axial space harmonics. Secondly, the electric field is supposed uniform in the azimuthal coordinate, neglecting the higher azimuthal space harmonics. Thirdly, the azimuthal electric field is supposed zero, neglecting the transverse electric field. Finally, the electric field is supposed equal before perturbation and after. Also, the affection of the perturbation rod’s dimension and relative permittivity on interaction impedance