The invention relates to a method for detecting a parasitic loop of a transformer station direct current system, and belongs to the field of electrical measurement. The method comprises the following steps that: a current sensor is set on a power output loop of the direct current system; the sensor measures the difference current of the system; a secondary coil of the sensor is used as a inductance coil of an active oscillator; a pulse counter measures the oscillation frequency of the active oscillator, and transmits the measured result to a microprocessor circuit for logic comparison; the microprocessor circuit sends an alarm signal if the change of the oscillation frequency of the active oscillator reaches a preset value; one loop of direct current power supply is switched off if the alarm signals appear on the two loops of power output loops at the same time; and the parasitic loop exists between the two power output loops if the alarm signals disappear on the two loops of power output loops at the same time. The function of detecting the parasitic loop fault of the direct current system is realized by measuring the oscillation frequency of the active oscillator; both an isogenous parasitic loop and a heterogenous parasitic loop can make an alarm. The method can be widely applied in the field of direct current system monitoring of a transformer system.