A sub-type current transformer belongs to the current transformer of a power system and is suitable for the current measurement of each level of the high-voltage power system so as to solve the problems of the existing electronic current transformer that the structure is complicated; the temporary failure current can not be measured accurately and the long-term stability is poor. A high-voltage-side sensing unit and a lower-voltage-side signal processing unit of the current transformer are connected by a signal optical fiber; the sensing unit comprises a Roche coil, a sampling resistance, a low-pass filter circuit, an amplifying circuit, a pressure-frequency conversion circuit and an electro-optical conversion circuit; the signal processing unit consists of a photoelectric conversion circuit, a pulse counter and a microprocessor. The current transformer has the functions of analog-to-digital conversion and ideal integrator. The sensing unit needs no simulation or digital integral circuits and also needs no sampling synchronous signals, thus largely simplifying the structure and the hardware circuit of the transformer and enhancing the reliability of the current transformer.