The multi-group constant processing software Ruler was developed based on the modern software engineering development standard. The well-proven models and methods for generating multi-group constants were used, including resonance reconstruction, Doppler broadening, thermal scattering processing, group cross section and scattering matrices computing, as well as resonance self-shielding treatment. The Ruler consists of 11 modules, which are the main driver, data input and output module, public mathematics module, system common subroutine module, data mode converting module, resonance reconstruction module, Doppler broadening module, unresolved resonance self-shielding module, thermal data generating module, neutron multi-group constants producing module and WIMS-D format data library preparing module, respectively. The validation of Ruler was done against the multi-group generation and benchmarks. It shows that the computational accuracy of Ruler agrees well with that of the NJOY99 code system, but Ruler is much more advanced in the aspects of the computing speed, the maintainability and the extendibility.