Abstract: Venus 1# assembly, which is established for ADS (accelerator driven sub-critical system) research in China, is the first sub-critical assembly consisting of both fast and thermal neutron spectra zones internationally. Fission rate of the assembly was calculated with MCNP code. The calculated result was referenced to determine the positions of detectors in fuel elements. Meanwhile, the measuring principle for fission rate by solid-state nuclear track detector was introduced, and the measuring results were also given. The measurement results show that the axial fission rate at the end close to the reflector is higher than that at the end without reflector, and the effect of reflector increases fission rate. The results provide important data for determining the fission power of thermal neutron spectrum zone.