Abstract: For optimization design of a neutron multiplicity counter and forming a guidance of the experiment, we describe a detailed method using Monte Carlo codes to generate neutron time pulse chain for stochastic simulation. A new fitting algorithm to calculate the die-away time was proposed. The applicability of calibrated parameters using 252Cf to plutonium samples was evaluated on the big cavity detection system, and the detection efficiency bias was calculated. Using the optimizing principle, the gate width in MSR algorithm was given for several plutonium samples. The curve of expected assay precision (RSD) vs 240Pu effective mass was given. The simulation results showed that the difference of detection efficiency between 252Cf and Plutonium was 3%. No difference was observed in gate fraction between using ratio method and formula method. With a measure time of 1000 s, the big cavity detection system had a RSD of about 4% for kg level plutonium metal. Those results will play an important role in determining necessary measurement time in plutonium measurement.