A plasma waveguide switch based on multipactor effect for switching low/high power microwave is proposed. No external triggering and no actuator are needed for the switch. Through controlling the electron flow formed on the waveguide sidewall based on field-induced electron emission and secondary electron multipactor, the high power capacity, low loss, and rapid activation time are realized in the microwave switch simultaneously. The power loss is about −20 dB, turn-on time is 5 ns, and the power capacity of hundreds of megawatt can be achieved. The switch has been used for backward wave oscillator injection locking.