A new-type optoelectronic current transformer based on digital modulation to replace the traditional CT is researched and designed in this paper. High-accuracy miniature CT and A/D modulation are adopted to realize the high-accuracy. Optical fiber communication technology is introduced, and the advantages and design scheme with phase encoding for OECT are presented. FPGA device XCR3032XL, low power-consumption ICs and EDA design technology are selected to satisfy the low power-consumption property. DSP calculates the ratio error of OECT; and the real-time property of data processing can also be ensured. By test running in laboratory, design requirements of the proposed OECT are achieved, and error sources are analyzed in detail.