High power traveling wave tubes (TWTs) usually use compound pipe shells to enhance the integration and heat dissipation characteristics of high frequency system. When broadband TWT adopts the compound pipe shell high frequency manufacturing process, the vane load contains ferromagnetic material (pure iron), which makes the transverse magnetic field component of the focusing system become larger, radial and angular magnetic field components being non-homogeneous, resulting in the difficulty of electron beam focusing. This paper studies the causes of transverse magnetic field generation in periodic permanent magnet (PPM) focusing system and establishes theoretical model and simulates the transverse magnetic field component’s effect on the morphology of electron beam. The simulation results are consistent with the theoretical model. The optimized combination of shape and number of vanes based on the theoretical model shows that the 9 tooth-shaped loaded vanes can maintain slow-wave circuit parameters while reducing transverse magnetic field component of the focusing system, improving the focusing performance of electron beam.