The invention discloses a bypass isolation device for a cascaded converter system, a cascaded converter system and a control method. The bypass isolation device comprises: first input, second input, first output, a second output terminal and a control terminal, an alternating current input is connected to the first input terminal and the second input terminal. The first output terminal and the second output terminal output AC input, the conductive connection between the first input terminal and the second input terminal is in a disconnected state, the first input terminal and the first outputterminal, the second input terminal and the second output terminal are respectively in a conductive connection state, and the control terminal receives the control signal; and the bypass isolation device disconnects the output lines of the first output terminal and the second output terminal after forming a conductive connection between the first input terminal and the second input terminal according to the control information received by the control terminal. The invention can carry out on-line maintenance on the converter module when the cascaded converter system is in the running state, improve the system reliability and ensure the safety of maintenance.