The equivalent circuit of the ICI detector has been established in this paper. The rise time of the ICI detector is 0.59 ns

which is calculated with the distributed parameters. The experiment for measuring the time response of the ICI detector is carried out on the multi-purpose radiant source. The time response obtained by the experiment is (0.677±0.144) ns

which agrees well with the calculation result. It is shown that the ICI detector has sub-nanosecond time response property and can be used to measure the rapidly changing pulsed gamma-ray flux.