Abstract: According to the electron beam parameters and Brillouin magnetic field formula and the axial magnetic flux density peak empirical formula of the periodic focusing system, the axial peak magnetic flux density of the permanent magnet focusing system is calculated. The simulation model of the permanent magnet focusing system of the four-beam travelling wave tube is established by Ansoft Maxwell 3D software. The structural dimensions of the model are parametrically analyzed, the influence of the structural dimension of the periodic permanent magnet focusing system on the magnetic field in the tunnel is studied, the optimal size matching of the magnetic focusing system is determined, and the periodic permanent magnet focusing system of the four-beam traveling wave tube is optimally designed. The axial magnetic flux density peak B_z=309 mT, the transverse magnetic flux density B_t=2.86 mT, B_t/B_z=0.92% on the central axis of the electron beam tunnel.