Abstract: Coupled simulation of deuteron or triton ionization and secondary fusion reaction was studied with a Monte Carlo tool named RSMC (Reaction Sequence Monte Carlo). The detail history and condensed history methods were employed for ionization simulation. Fusion cross sections of deuteron and triton were adopted from ENDF or TENDL. The “forced particle production” variance reduction technique was also employed to improve the simulation efficiency. As a validation, three types of examples were introduced, including neutron depth profiling, accelerator based mono-energy neutron source, and thermal-to-fusion neutron convertor.