Abstract: Deuteron ion transportation, fusion reaction sampling and neutron transportation simulation are included in the numerical calculation of accelerator driven neutron generator. As well known, the nuclear reaction cross section is much smaller than Coulomb cross section of charged particle transportation, and the mean free path of nuclear reaction is far longer than the depth of target, compared to deuteron ion transportation, the direct sampling efficiency of fusion reaction is almost zero. In this paper, a "forced" fusion arithmetic is introduced and embedded into MCNPX program. By the "forced" fusion arithmetic, fusion reaction must happen for every source deuteron ion. The position, energy and flight direction of deuteron ion that interact with tritium are sampled from all energy steps. The position, energy and flight direction of fusion source neutron are sampled according to that of deuteron ion. The deuteron ion and neutron coupling transportation can be calculated with the revised MCNPX.