In order to ensure the smooth development of online monitoring of an electric power system, and promote the development of self-power and intelligence of a current sensor, a type of current measuring sensor with self-power supply function is designed. The two functions of measuring and supplying power are realized using one transformer. This paper deduces the mathematical relationship of voltage and current between primary side and secondary side of the transformer, and the situations before and after the rectifier bridge are taken into account. The general design idea of the current measuring device is given. Finally, the correctness of different measurement models is verified by experiment, and the test results of the load change are analyzed. This work is supported by National Natural Science Foundation of China (No. 51707117) and Science and Technology Project of State Grid Anhui Electric Power Co., Ltd. (No. 52120017001G).