Abstract: Multi-group constants are the basis of the deterministic physics calculation, and the accuracy will directly affect the subsequent calculations. The methods of multi-group constants processing are developed rapidly, and the study on the processing methods and relevant code developing have great importance. Multi-group constants contain multi-group cross sections, transfer matrices, fission constants, etc. Based on the evaluations and the reaction rate conservation theory, the neutron slowing down equation is solved by the recursive hyper-fine group method and the nuclide mixing method is proposed. A module named Group_collapse for processing multi-group constants is developed. Numerical results show that the theories are correct, and the multi-group constants can be available for the deterministic physics codes which are based on the conventional two-step method and the one-step method.