The invention provides a method for detecting partial discharge of a transformer. The method comprises steps of: installing a current sensor at a grounding lead wire place or an earth lug place of an external iron core of the transformer; converting a current signal detected by the current sensor into a voltage signal; converting the voltage signal into a partial discharge signal; converting the partial discharge signal into a two-dimensional spectrogram; and comparing a value of a statistical characteristic corresponding to the two-dimensional spectrogram with a value of a classic statistical characteristic and judging different discharge modes in the transformer. According to the invention, detection is performed without changing original connecting wires of the transformer; real-time online monitoring of the partial discharge of the transformer is achieved without affecting safe and stable operation of the transformer; and compared with the normal method, the method provided by the invention can quite precisely monitoring partial discharge conditions in low-level and high field intensity regions of the transformer.