Overview: It is an important part for studying lightning to measure lightning current. The time domain waveform of lightning current is a fast pulse. The traditional method of measuring lightning current is based on the principle of electromagnetic induction. The traditional current sensor is bulky, heavy and difficult to insulate. It is inevitably limited by the traditional measurement method, such as magnetic saturation. All-fiber optical current transformer (AFOCT) has many advantages over traditional electromagnetic sensors, such as large dynamic range, small size, strong anti-electromagnetic interference capability, high safety, good insulation and light weight. Consequently, this paper studied an AFOCT for measuring lightning currents. Firstly, the basic principle and structure of the AFOCT are introduced. The current transformer is based on Faraday magneto-optical effect and Ampere's law. It adopts reflecting optical path system and closed-loop detection scheme. The scheme has the advantages of large dynamic range and high detection precision. Secondly, the performance including the response speed, measurement accuracy and measurement range was tested in the laboratory. The typical lightning current is measured by the AFOCT. The tested current includes the waveform of lightning current component A and the lightning current 8/20 μs waveform. The waveform of lightning current component A is specified in the national military standard GJB1389A, and it is also used by the US military standard widely. It is the waveform required for lightning protection test of equipment. The waveform of lightning current 8/20 μs has relevant regulations in IEC standards and national standards, and it is mainly used for injection testing of lightning protection devices. Therefore, these two waveforms were used in the test, and they are respectively generated by two pulse current generators with adjustable output levels. The results of measuring the waveform of lightning current component A by the AFOCT are used to obtain the sensor's ability to detect small currents. The results of measuring the waveform of lightning current 8/20 μs by the AFOCT are used to obtain the sensor's ability detect large currents and the upper limit of the range in different turns. Through the testing of the AFOCT, the measurement accuracy and dynamic range of the sensor are obtained. The results show that the response speed of the sensor is in microsecond. The measurable range is over 1 kA~100 kA. The dynamic range is greater than 40 dB and the measurement error is less than 5%. The measurement waveform of all-fiber optical current transformer coincides with that of standard Pearson current probe. The paper provides a new method for lightning current measurement.