Abstract: Fast neutron multiplicity counting (FNMC) analysis method can effectively measure the properties of samples. Based on the fourth-order FNMC analytical equations, a set of three-layer fast neutron multiplicity counters with six liquid scintillators per layer was constructed for Geant4 simulation, and the values of related parameters were determined. Metal Pu sample with 1 cm iron, aluminum, carbon, and stainless steel packaging material was externally simulated, and the sample satisfied the assumption by the equation adaptive analysis. The measurement parameters such as detection efficiency and multiplicity counting rate were simulated. When the mass of Pu sample is less than 500 g, the increase of sample solution mass deviation is less than 1.20% with carbon as packaging material, and the influence of iron material and stainless steel material is less. According to the measurement results, the self-multiplication factor was corrected for the sample without shell, and the third-order polynomial fitting equation was obtained and the goodness of fit is 0.933. The corrected solution mass deviation of sample with mass less than 1 kg is less than 6.00%. The results show that the medium-heavy metal with thickness of 1 cm has little effect on the measurement of Pu samples. The combination of the fast neutron multiplicity counter and the coefficient correction method can achieve more accurate measurement of the sample properties.