Abstract: In this paper, the quasiperiodic boundary conditions in the computer code MAFIA was used to determine the effect on dispersion and onaxis interaction impedance of helical slowwave structure. The Finite Element Method softwareANSYS was used to simulate the helix pressed distortion. And then the effect of pressed helix in helical slowwave structure on TWT coldtest characteristics was analysed in detail. The results show that both dispersion and onaxis interaction impedance of helical slowwave structure increase due to helix deformation generally, when the radial relative deformation is less than 1%.While radial relative deformation is more than 1%, the onaxis interaction impedance will decrease.