Neutron Flux Monitor, a key diagnostic system in International Thermonuclear Experimental Reactor, may provide for reading a series of important parameters in fusion reaction process. We designed an important part of its main electronic system — Automatic Gain Adjustment Campbell Integrator (AGACI), expanding the detecting neutron counting rate of 10⁴–10⁸ cps (counts per second). The total gain of AGACI in five files is divided into 1, 10, 100, 1000, and 10 000 times, and can be automatically adjusted. The linear correlation coefficient (R) is more than 0.9999 in AGACI working range.