An opt-electric transmission system of low power consumption that consists of the up- and down-fiber for the electronic current transformer（ECT） is designed. The command data signals can be transmitted via the same up-fiber that is used to transmit power from the secondary power supply to the primary converter containing sensors and associated circuitry. The up data transmission is done during the brief interruption of optical power supply， which does not affect the steady electrical power supply. The command data can be picked off by a simple comparator circuit added to the power-conversion circuit in the primary converter. The improved circuitry driving the fiber transmitter in the primary converter works reliably with only a few mWs average power consumption， and the total power consumption of the primary converter can be limited to merely 40 mW. So the output of the optical power-supply is considerably cut down. The experimental results show that data can be transmitted at a baud-rate as high as 200 kbit/s from the secondary to the primary converter， and 2 Mbit/s in the opposite direction. The baud-rate will meet the communicating demand of the CT. Together with the digital data processing techniques the transmission system can cut down power consumption and improve the measuring accuracy if applied in ECTs.