The invention relates to a differential protection and relay protection device of a rectifier transformer set, and in particular relates to a differential protection method of a large-power rectifier transformer set. The method is characterized by comprising the following steps: 1, connecting voltage current of the first side, voltage current of the third side, a current signal at the secondary side, namely, the rectifier transformer side, into a collector, converting current voltage signals of FT3 formats, and then connecting to a digital combiner; 2, combining the voltage current signals of FTE formats at each side based on a plurality of paths, converting into 9-2 messages, synchronously processing the current and voltage information input by the collector, and then sending to a regulating transformer protection device through optical fibers; 3, receiving the current signals at each side of the digital combiner through the regulating transformer protection device, then calculating the differential current, and thus realizing related calculation and judging. With the adoption of the protection method, the configuration of the relay protection of the rectifier transformer set is improved, and the recognizing capacity of the protection device for the inner and outer faults of a rectifier transformer area and the fault of a rectifier set is further improved.