A novel current measuring principle based on hollow coils is proposed，on which the current is measured through several coils placed around conductors according to certain rules.Firstly，based on the electromagnetic induction law and Biot-Savart law，the relationship between induced voltage of single coil and the conductor current is derived.On this basis，the method of measuring the single-phase current and that of measuring the three-phase current are presented，respectively and verified by simulation and experimental results.Finally，the measurement deviation caused by actual factors is analyzed.And the relative error curve of the sensor in the coil installing process is obtained through simulation.