Abstract: This paper studies that the conductance decrease of distributed lossy material can increase wave lossy and heighten threshold current for the absolute instability and start-oscillation length for the gyro-baekward-wave oscillation. However, the decrease also reduces linear growth rate, axial power and high frequency field gain of gyro-TWT. The paper also shows the design of cylindrical waveguide structure gyro-TWT, using graphite as the distributed lossy material, and the factors are as follows ： electrons current I0 = 10A, interaction length L = 10cm. The result can effectively make a compromise between power, gain and instability suppression, ensuring the stable operation of the gyro-TWT.