Abstract: Neutron multiplicity counting (NMC) has been widely used in many fields related to non-destructive nuclear material measurement, such as nuclear material protection control and accounting, arms control verification and so on. Developing simulation tools, and carrying out numerical experiments are important means in the research on the application of NMC, which also become one important area of neutron transport simulation. This paper presents a new method to calculate NMC by sequential detection events simulation. By realizing the on-line NMC calculation, based on JMCT particle transport simulation code, the program to simulate neutron multiplicity counting, JMCT_NMC, was developed. This paper also presents some results of NMC calculation examples to test the code, and comparison of the time consumption of JMCT_NMC and traditional method. In JMCT_NMC code, by getting rid of the large data storage requirement, it is possible to simulate neutron multiplicity with large event number and high calculation efficiency.