An open rhombus metamaterial structure is proposed to suppress the parasitic mode oscillations and improve the beam-wave interaction efficiency of a U-shaped meander-line slow wave traveling wave tube. The effect of the metamaterial on the surface E-field enhancement of the beam-wave interaction and the suppression of the parasitic oscillations were discussed through the combination of the circuit design with the numerical simulation optimization by considering the resonant performance of the metamaterial unit, the coordination of the matamaterial with the slow wave structure, the realizability and simplicity of the circuit structure. The simulated results of a Ka-band U-shaped meander-line slow wave traveling wave tube show that this metamaterial is effective in suppressing the parasitic oscillations and improving beam-wave interaction efficiency, which is of great significance of improving the stability of this kind of traveling wave tube.