Abstract: The paper concernes in the neutron source multiplication method research in nuclear critical safety. Based on the neutron diffusion equation with external neutron source the effective sub-critical multiplication factor k_s is deduced, and k_s is different to the effective neutron multiplication factor k_(eff) in the case of sub-critical system with external neutron source. The verification experiment on the sub-critical system indicates that the parameter measured with neutron source multiplication method is k_s, and k_s is related to the external neutron source position in sub-critical system and external neutron source spectrum. The relation (between) k_s and k_(eff) and the effect of them on nuclear critical safety is discussed in the (paper.)