The invention discloses a power plant relaying protection device remote diagnosis system and diagnosis method. The power plant relaying protection device remote diagnosis system comprises a monitoring terminal, a monitoring management machine, a first data transmission line and a plurality of second data transmission lines; the first data transmission line comprises a first switch, a second switch and an optical cable connecting the first switch and the second switch; the first switch is connected to a relaying protection device inside a low voltage chamber through a cable; the second switch is connected to a supervising management machine through the cable; the second data transmission line comprises a first photoelectric converter, a second photoelectric converter and a cable connecting the first photoelectric converter and the second photoelectric converter; each first photoelectric converter is connected to the corresponding relaying protection device inside a power generating chamber through a serial data line; and the second photoelectric converter is connected to the supervising management machine through the serial data line. The power plant relaying protection device remote diagnosis system and diagnosis method can remotely monitor whether the relaying protection device is working normally and find faults of the relaying protection device timely.