Abstract: Based on the probability method that neutron will interact with the medium in nuclear assembly, the equation that governs neutron initiation probability W( r, E, Ω, t) was derived. On the assumption of static and slightly prompt super-critical condition, the formula of initiation probability of one neutron was obtained using second-order approximations and perturbation method. With the formula and nuclear parameters of Godiva-Ⅱ, Caliban and CFBR-Ⅱ burst reactors, the initiation probabilities of one neutron are 0.000 32, 0.000 54 and 0.000 27 respectively, and the experimental data attained on Godiva-Ⅱ,Caliban were used to compare with and discuss the theoretical results.