The paper first briefly reviews the neutron source multiplication method and then presents an experimental study that shows that the parameter measured by the neutron source multiplication method actually is a subcritical effective neutron multiplication factor ks with an external neutron source, not the effective neutron multiplication factor keff. The parameters ks and keff have been researched for a nuclear critical safety experiment assembly using a uranium solution. The parameter ks was measured by the source multiplication method, while the parameter keff was measured by the power-raising period method. The relationship between keff and ks is discussed and their effects on nuclear safety are mentioned.