In this work,we developed a numerical analysis-associated experiment method to determine the effective multiplication factor k_(eff),which is difficult to obtain directly from conventional neutron source multiplication(NSM)method.The method is based on the relationship between k_(eff),subcritical multiplication factor k_s and external neutron source efficiency Φ~* in the subcritical system.On basis of the theoretical analysis,the dependence of k_s and Φ~* on subcriticality and source position was investigated at the Chinese Fast Burst Reactor-Ⅱ(CFBR-Ⅱ).A series of k_s were measured by NSM experiments at four subcritical states(k_(eff) = 0.996,0.994,0.991 and0.986) with the ~(252)Cf neutron source located at different positions(from the system center to outside) at each subcritical states.The Φ~* was obtained by Monte-Carlo simulation for each condition.With the measured k_s and calculated Φ~*,k_(eff) of the subcritical system was evaluated with a relative difference of < 1%between values obtained by the improved method and by positive period method.Especially,the relative difference of < 0.18%with the source located at the system center. In this work, we developed a numerical analysis-associated experiment method to determine the effective multiplication factor k_ (eff), which is difficult to obtain directly from conventional neutron source multiplication (NSM) method. The method is based on the relationship between k_ ( off), subcritical multiplication factor k_s and external neutron source efficiency Φ ~ * in the subcritical system. On basis of the theoretical analysis, the dependence of k_s and Φ ~ * on subcriticality and source position was investigated at the Chinese Fast Burst Reactor-Ⅱ (CFBR-II). A series of k_s were measured by NSM experiments at four subcritical states (k eff (eff) = 0.996,0.994,0.991 and0.986) with the ~ (252) Cf neutron source located at different positions system center to outside) at each subcritical states.The Φ ~ * was obtained by Monte-Carlo simulation for each condition .With the measured k_s and calculated Φ ~ *, k_ (eff) of the subcritical system was evaluated with a relative difference of <1% betw een values ​​obtained by the improved method and by positive period method .Especially, the relative difference of <0.18% with the source located at the system center.