The integrity of insulation structure was of great importance for the longterm reliable operation of windgenerator. Based on the analysis of the structural features of largesize directdrive permanentmagnet windgenerator, a new operation process of sealing the iron core slot during stator winding inserting course was put forward to improve the filling effect of insulating resin. The iron core slot of the product made by this process was full filled by insulating resin. Test data such as insulation resistance, dielectric loss factor and breakdown voltage were improved obviously compared with those of the original structure. The results indicate that integrity and reliability of insulating structure are assured effectively with the sealing process.