A numerical method of the statistics fluctuation problem of fission chains is studied for fission system driven by a weak neutron source.Constructing model of numerical simulation,analysis of numerical method,examinations of numerical simulation,demonstrating of numerical simulation for a kind of problem have been done.Examinations of numerical simulation for special example indicated that so long as the order of the system of numerical siumlation (N) is large sufficiently,numerical solutions satisfy normalization (conservation) law、population growth exponential law,and approach to the exact analytic solution.An one-diminsional equivalenly modelling numerical method is presented for statistics fluctuation problem of neutron fission chains in unsteady fission system.