In order to solve the problem of energy supply on the high voltage side of the active electronic current transformer (ECT), a signal coding method based on a three-state gate circuit is proposed. The sampling clock signal and sampling data signal of the analog-to-digital converter (ADC) are encoded as a four-level signal by the three-state gate circuit, and then directly drive the ultra-high-brightness LED to be converted into optical signals and transmitted to the low-voltage side for decoding. This method requires only a one-way photoelectric transmission system to realize three-wire SPI communication, which can reduce the power consumption of the high-voltage side to less than 5mW, so it can be selected as a disposable high-capacity battery power supply to solve the problem of the high-voltage side of the energy supply. In addition, for the problem of four-level signal distortion affecting signal recognition and decoding found in the experiment, a recognition method based on support vector machine (SVM) is proposed. The results show that the recognition correct rate can reach 99.25%, and the recognition of a single pulse takes only 11.46μs, which can meet the signal recognition requirements of ECT.