The invention relates to a trigger circuit used in high power SCR industrial frequency ac switch which is characterized by a switching circuit comprises resistances R1, R2, R3, diodes D1,D2,D3, capacitance C1, triode BG and a reverser U1A. The pulse osc which comprises resistances R4, R5, R6, diodes D4, D5, capacitance C2, reversers UIB, UIC is transited by D3. The pulse osc connects the driving circuit composed by reversers UID, UIE, UIF, resistances R7, R8, R9. The three circuits outputs parallely at the same time to trigger the SCR. The circuit is simple, reasonably designed and is more simplified in use and operation. The invention which is of low cost and low energy consumption and is not easy to interrupt other circuits or be interrupted by other circuits can be adjusted, has a stable and reliable performance and a wide scope of application.