Abstract: A phase switch (energy switch) comprising a three-cavity system (an end-coupled cavity+side-passed accelerate cavity+an end-coupled cavity) and a separate single couple cavity is disclosed. The phase shift between the adjacent accelerate cavities is ? when the three-cavities system is disordered (state ‘0’); and a microwave pass through the three-cavities system to the adjacent accelerate cavities, the phase between the adjacent accelerate cavities is change to 2? (or 0) when the single couple cavity is disordered (state ‘1’). When the state 0 changes to state 1, the field phase in the structure behind the system is changed to ?, thereby to switch the phase. In the two states, the entire structure operates in ?/2 mode, that is very stable. That is very important for the medical accelerator. The detaining components have been moved outside the cavity when the single couple cavity or the three-cavity system is in the operate state, without warring about high frequency breakdown.