Abstract: The paper presents a new method to calibrate detection efficiency of delayedneutron detector for monitoring breakage of fuel elements in nuclear reactor. The device consists of three parts: loop of test installed on the nuclearreactor, simulating failed fuel element and fission track detector. Under same condition, at first the counts of delayed neutron are measuredusing delayed neutron detector, then the counts of delayed neutron are measuredusing fission track detector. Results show that the flux rate of delayed neutron equals 4.8 n/cm~2·s, andthe detection efficiency equals 8.1%.