Abstract: It’s important to know the neutron energy spectrum at the interface between the fast and thermal neutron regions of the accelerator driven coupled fast/thermal spectrum device—Venus 1#. The neutron energy spectrum was measured by the multifoil irradiation method at the interface between the fast and thermal neutron regions. The neutron spectrum unfolding was done by using SANDⅡ and MIST computer code. The calculated spectrum, which was used as preinformation in the adjustment procedure, was obtained though the MCNP code. In this calculation, the neutron source, the coupled fast/thermal neutron material region as well as its surroundings were modeled in threedimension. A comparison between the calculated and measured neutron spectra shows well agreement, which indicates the MCNP model correctly simulates the Venus 1#.