The multipactor effect limits the performance and power capacity of spacecraft microwave systems. This phenomenon of secondary electron multiplication occurs under specific radio-frequency operating conditions during the secondary electron emission process on material surfaces within microwave devices. Multipactor deteriorates the performance of the components and, in severe circumstances, ca n even lead to device or spacecraft failure. Alumina ceramics possess favorable dielectric properties, high hardness, excellent thermal insulation, and low dielectric loss, making them widely used in high-power microwave systems, including components such as filters. However, the surface of alumina ceramics exhibits a high secondary electron yield (SEY), indicating that, in space environments, high-power microwave assemblies filled with alumina may trigger destructive secondary electron multiplication. In this work, a Ku-band single-pole double-throw microwave switch structure was designed, employing medium filling to reduce high-frequency losses. Its operational frequency range spans DC to 18 GHz. Through matched simulation optimization of shielded striplines, the microwave switch achieves matched transmission across an 18 GHz broadband range. After optimization, the switch exhibits in-band loss below 0.23 dB and isolation exceeding 90 dB. Simulation studies were conducted on the evolution of secondary electrons within microwave switches under varying input power conditions, with the internal structure configured to utilize lossless alumina as the filling medium. Results indicate that after filling with untreated alumina media discs, the switch's multipactor threshold reached 10874 W. Two experimental methods were employed to modify the alumina surface for SEY tuning: laser etching to create a microporous structure with 67.24% porosity, and sputter deposition of a TiN functional film using N₂∶Ar gas flow ratio of 7.5∶15. Test results demonstrated that applying either technique independently reduced the SEY on the alumina surface from 3.56 to 1.71 and 1.99, respectively, while the first critical energy E_P1 increased from 26 to 70 eV and 40 eV, respectively. Simulation results indicate corresponding microwave switch multipactor thresholds elevated to 12624 W and 12374 W, respectively. By combining the two techniques, the SEY on the alumina surface was further reduced to 1.13. The first critical energy E_P1 increased from 26 to 161 eV, and the microwave switch multipactor threshold was elevated to 13124 W, marking an improvement of approximately 0.82 dB over the original device. In practice, secondary electron multiplication is a dynamic process in which electrons move under continuously varying radio-frequency electric field. When considering the influence of the radio-frequency electric field on electron trajectories, it is found that the field promotes the escape of secondary electrons from the microstructure, thereby weakening the microstructure’s ability to suppress multipactor. Concurrently, the accumulation of surface charge on the alumina surface caused by secondary electron emission affects the energy of both incident electrons and emitted secondary electrons, thereby influencing the surface SEY and multipactor thresholds. Fabricating microstructures on the alumina surface can reduce the potential level of the positive surface charge. The SEY of a charged surface irradiated by 2000 pulses can raise the multipactor threshold of the device from 10874 to 12624 W. This indicates that the charged surface may promote an increased multipactor threshold to some extent. Taking into account the electrostatic field at surface saturation (E_DC), the Hatch-Williams model was modified, and the role of the electrostatic field induced by surface charges in the dielectric in raising the multipactor threshold was qualitatively demonstrated using theoretical equations. This work holds significant engineering application value for enhancing the reliability of high-power microwave devices.