To enhance the rapid response ability of manufacturing system, the machine tools' modular design oriented to reconfiguration was analyzed. Then the general idea of machine tools' reconfiguration oriented to process-capability was presented. The machine tools' reconfiguration arithmetic based on modularization was researched mostly in such aspects as ability similarity account, relativity analysis, module reconfiguration of lacking ability parameter, module reconfiguration of redundancy ability parameter, ability comparing, module displacing and ability checking up. The arithmetic was validated by the configuration and reconfiguration of machining center, which indicates the arithmetic is feasible and effective.