Microwave plasma thruster (MPT) is a kind of electrothermal thruster. Inside its cylindrical cavity, the plasma process, microwave electric field distribution, and TM₀₁₁ mode resonant state are important factors affecting the performance of MPT seriously. According to previous MPT formed through continuous regulation in the resonant sate of cylindrical cavity, the research is needed on a newly fixed and simple MPT, which will simplify the resonant state regulation and lays an important foundation for further study. Therefore the plasma process is analyzed to find the optimal gas discharge condition, and the microwave electric field intensity and power density distribution inside the cavity running in TM₀₁₁ resonant sate are calculated to analyse how the parameters are influenced by the cavity dimensions. The resonant state is finely regulated to study how it is influenced by the dimensions of cylindrical cavity and microwave coupling probe with ball and half ball structure. The results of theoretical analysis and calculation show that the discharge power of helium gas is the lowest under the condition of 489 Pa and when the ratio of length to diameter is greater than 1, the microwave electric density distribution inside the cavity is beneficial. Owing to the appropriate length and radius of microwave coupling ball probe, the experiment on resonant state regulation shows that the shortest cylinder cavity is in the optimal resonant sate, with a resonance frequency very close to 2.45 GHz. The helium discharge experiment proves that the cavity and matching ball probe enable high microwave utilization and easy helium gas discharge, and the structure scheme is correct and reliable.