During the cathode preheating process of a small amount of space traveling wave tubes(TWTs), there is a fault of periodic fluctuations in the heater current. In response to this problem, the range of fluctuation of heater current is estimated by creating the equivalent circuit of heater, and the natural frequency of heater double-helix structure is measured by using the laser vibration, thereby the localization of the problem is completed, and the mechanism of period fluctuation of heater current is analyzed, and the conditions under which the periodic fluctuation occurs are clarified, and the solutions are put forward. Meanwhile, theoretically, the qualitative analysis is conducted on the influence of heater current on the reliability of the space TWTs, and the correctness of the conclusion of reliability influence analysis is verified through experiments.