It is an important part for studying lightning to measure lightning current. Consequently, this paper studied an all-fiber optical current transformer for measuring lightning currents. Firstly, the basic principle and structure of the all-fiber optical current transformer were introduced. Then, the performances including the response speed, mea-surement accuracy and measurement range were tested in the laboratory. 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.