To obtain the optimization design of neutron shielding by iron/borated-polyethylene composite structure, we built a neutron shielding model of two layers of iron and borated-polyethylene. For neutron with various energies, the neutron transmission coefficients of iron/borated-polyethylene composite shield with different thicknesses were obtained by using Monte Carlo method. The simulation results show that, when neutron energy is 14 MeV and the total thickness of the composite shield is 40 cm, 60 cm and 80 cm, the optimal thickness ratio of iron to borated-polyethylene is about 0.7: 0.3, 0.725: 0.275, and 0.75: 0.25, respectively. The optimal thickness ratio of iron to borated-polyethylene is usually higher than the case of iron/polyethylene composite structure.