The invention discloses a device for replacing a relay protection device in a switch cabinet and an assembly method of the device. A human-computer interaction module of the relay protection device for being placed is separated from a case body; the case body is arranged in the cabinet; the human-computer interaction module is arranged outside the cabinet; the case body is connected with the human-computer interaction module through an existing cabinet door opening; a communication connector passes through the existing cabinet door opening to connect the human-computer interaction module with the case body in a communication manner; and a backboard terminal on the case body is connected with an existing secondary circuit on the switch cabinet through a terminal adapter. Through the replacement scheme, seamless replacement of the relay protection device is achieved without exchanging a switch cabinet door or forming an opening in the cabinet door again or changing terminal connection, so that the cost is reduced, the production efficiency is improved, the construction risk is reduced and the on-site power supply reliability is improved.