Abstract: This paper puts forward a field calibration method of differential ring for nanosecond pulse current measurement after analyzing the problems in calibration. To resolve the low signal-to-noise ratio and low reliability issues, the pulse charging principle of pulse forming line is analyzed. The relation between calibration result and charging capacitance, charging inductance, pulse forming line voltage and pulse forming switch breakdown voltage is also analyzed. Field calibration of the differential ring for pulsed electron beam current measurement is conducted on the Flash Ⅱ accelerator. When the average breakdown voltage of the front gap switch is 25 kV, the calibration current is 1.3 kA, the average sensitivity of the differential ring is 9.311010 with a deviation of 0.151010.