the time character of delayed neutrons is considered. The method of computing transportation of delayed neutrons is studied. Technology and method of detecting nuclear warheads by counting delayed neutrons are discussed. The results show that a nuclear warhead could be detected by counting delayed neutrons. But
this method could not discriminate the real warhead from the fake one that contains only natural uranium if the design information of the warhead is unknown beforehand. So the joint use of this method with others will be necessary.