The film system structures of Co/Ti multilayer film were designed optimally at the L-absorption edge of Ti(452.5 eV) to meet the requirements of a multilayer mirror at the ‘water window’ energy region between 280 eV and 540 eV. The reflectivity of the multilayer film was calculated at different interface roughness conditions and it shows that the interface roughness has greater effect on the reflectivity of the multilayer film. Co/Ti multilayer films were deposited on a Si substrate by DC magnetron sputtering method and the quality of these films was improved by adding nitrogen gas into original argon gas in magnetron sputtering. Finally, the multilayer film structure was measured by a Grazing Incident X-ray Reflection (GIXRR) method and a Transmission Electron Microscope(TEM), and the reflectivities in different nitrogen concentrations were measured by Soft X-ray Reflectivity(SXR) at Beijing Synchrotron Radiation Facility(BSRF). The results show that the reflectivity of the Co/Ti multilayer with nitrogen gas fraction of 5% is the highest among the samples, by which the reflectivity has been improved from 9.5% to 12.0%. The results demonstrate that the reaction sputtering with nitrogen gas improves the quality of Co/Ti multilayer films effectively.