Abstract: Based on the assumption that the guide magnetic field is infinite, the dispersion equation for a ripple slow-wave structure filled with plasmas was derived. Through a numerical simulation, the effects of plasma densitywere analyzed on the dispersion property when there is no an electron beam in a slow-wave structure filled with plasmas and on the frequency and gainwhen there is an electron beam in the slow-wave structure. The research result shows thatwith the raisingof plasma density, the operation frequencyofTM01 module increases and the gain of a slow-wave structure filled with plasmas decreases continuously.