Abstract: With wide-pulse neutron injection the fission chain fluctuates statistically.A calculation approach is presented, in which numerical solutions of the statistical distribution W_n equations are disassembled and then synthesized.It makes the computation suitable for generalized conditions (with arbitrary time distribution,zero time (t₀) injection probability distribution P(t₀) and number probability distribution S(n) etc.).The equivalent conditions in practical physical models are discussed and expanded from single-medium to multi-medium.In addition, the average computing formulae for required parameters in solving W_n equations are given as well.

Key words: wide pulse, outer neutron source, statistical fluctuation, fission chain