Multiplication-reactivity conversion factor is a key parameter in CFBR-Ⅱ to get reactivity at subcriticality by neutron source multiplication method.It was measured in many experiments,but the results were different.In order to explain this,we derive an expression of multiplication-reactivity conversion factor with transport equation.Aimed at CFBR-Ⅱ,multiplication-reactivity conversion factor in different reactivity states are calculated with Monte Carlo Method.It is concluded that multiplication-reactivity conversion factor is determined by delayed neutron fraction,importance ratio of external neutron source and fission neutron source,neutron leakage probability ratio of multiplication system and substitute system,detector efficiency ratio of multiplication system and substitute system.Multiplication-reactivity conversion factor varies with CFBR-Ⅱ's reactivity.