Abstract: In order to eliminate the energy loss and electromagnetic interference caused by corona discharge,the corona characteristics of different kinds of fittings and the selection of fittings under different voltage levels are studied. The corona discharge phenomena of several EHV AC substations are observed by using UV imager,and the fittings which are prone to corona discharge are found out. A three-dimensional finite element simulation model is established for fitting structure,and the surface electric field intensity distribution of different sizes of fittings under 500 k V and 750 kV voltage levels is calculated,and the structure with larger electric field intensity is optimized. The results show that the surface electric field intensity decreases with the increase of the pipe diameter of grading ring,shielding ring and the ball diameter of shielding ball. Combined with the control limit of corona field strength on the surface of fittings,the size selection of shielding ring of tension insulator string,grading ring of V-type insulator string and shielding ball of conductor under 500 kV and 750 kV voltage levels is proposed.