Abstract: The paper gives experiment theory and experiment method of neutron source multiplication method for site measurement technology in the nuclear critical safety. The measured parameter by source multiplication method actually is a sub-critical with source neutron effective multiplication factor k_s,but not the neutron effective multiplication factor k_eff. The experiment research has been done on the uranium solution nuclear critical safety experiment assembly. The k_s of different sub-criticality is measured by neutron source multiplication experiment method, and k_eff of different sub-criticality, the reactivity coefficient of unit solution level, is first measured by period method, and then multiplied by difference of critical solution level and sub-critical solution level and obtained the reactivity of sub-critical solution level. The k_eff finally can be extracted from reactivity formula. The effect on the nuclear critical safety and different between k_eff and k_s are discussed.