Taking the tree discharge fault of dry-type smoothing reactors in Suidong converter station of ± 800 kV Yunnan-Guangdong UHVDC transmission project as example, the reason of the accident was analyzed, based on the equipment inspection and the calculation results of 3D electromagnetic field software. The results show that there are some deficiencies of the design of smoothing reactor in the UHVDC transmission project. So this paper put forward the improvement measures and some suggestions for operation and maintenance.