Abstract: Neutron multiplicity technique is commonly used to measure and verify nuclear materials, especially for objects with heavy shields. Plutonium material has high spontaneous fission rate and passive measurement is available. Currently, there are many different measurement devices. However, uranium material measurement can only be performed by active method due to its low spontaneous fission rate. The existing active well type coincidence counter (AWCC) is capable of performing active neutron multiplicity measurement of uranium. But the detection efficiency is low and there are lots of accidental coincidence counts caused by the Am-Li neutron source. In order to improve the efficiency and the accuracy it is necessary to carry out comprehensive investigation on active neutron multiplicity measurement method. A neutron multiplicity measurement system was modeled based on Geant4 referring to the AWCC structure. The effects of different gate width and delay time on the measurement deviation were studied. The optimal gate width of the counter is 44 μs and the range of the gate width is suitable for about 1.5 times of the counter die-away time; after the delay time is greater than 3 times the counter die-away time, the relative deviation is significantly reduced. The influence of ²³⁵U enrichment on the result was also discussed. This work provides a reference for the design of the active neutron multiplicity counter.