The existing busbar energy extraction and laser energy supply methods on the high-voltage side of a light-transmitting current transformer suffer from the problems of power supply dead zones, low energy conversion efficiency, and insufficient service life. Thus an ultra-low power optoelectronic transmission system scheme based on four-level coding and weak light communication is proposed. First, a three-state gate circuit is designed to encode the sampling clock and data signal of the signal sampling module into a four-level signal. Then, the four-level signal is converted into an optical signal and transmitted to the low-voltage side through an optical fiber using an ultra-high brightness and ultra-low power LED. Finally, the signal is decoded at the low voltage side using a high-sensitivity avalanche photodiode detector for weak light detection and then input to the decoding circuit. The ultra-low power system is applied to the light-transmitting current transformer for experimental verification. The results show that it can meet the requirements of 0.2-level accuracy, and the average power consumption of the high-voltage side can be reduced to 3.91 mW. Because of the ultra-low power consumption of the system, the high-voltage side can adopt a reliable energy supply scheme such as a large-capacity battery or low-power photovoltaic system. This solves the problem of power supply and improves stability and reliability.