The magnetic ring at the wave port position of the TWT periodic permanent magnet focusing system usually adopts single open magnetic ring. Two different double-open magnetic rings were proposed because of adding waveguide impedance tuning branch at the wave port position. The magnetic field near the central axis of the double-open magnetic ring was analyzed by using the 3D electromagnetic simulation software Opera-3D, and the design method of the periodic magnetic focusing system of double-open magnetic ring was put forward. In order to verify the feasibility of periodic permanent magnet focusing system with double-open magnetic rings, an electronic optical system of E-band folded waveguide TWT was designed and tested. In the traveling wave tube test, the emission current of the electron gun was 83 mA, and the electron beam flow rate of the periodic magnetic focusing system with double-open magnetic rings was 99%.