The invention discloses a loop current monitoring-based line breakage judgment method and device of a rotor grounding protection secondary loop. The method comprises the steps of measuring a voltage of a power generator rotor and a ring current of a rotor grounding loop by a protection device, calculating grounding resistor of the power generator rotor, taking the situation that the ring current of the rotor grounding loop is smaller than an action threshold as a main criterion, taking the situation that the rotor voltage and the rotor grounding resistance are larger than respective action threshold as an auxiliary criterion, judging that the rotor grounding protection secondary loop is broken when the main criterion and the auxiliary criterion both are conformed, and acting to warn by protection after delaying time t. By the method, a line breakage fault of the rotor secondary loop can be identified, a user is reminded of maintenance, and the situation that the rotor grounding protection rejects to perform brought by the line breakage fault of the rotor secondary loop is prevented.