Abstract: The insulation structure of valve side lead-out is the key area affecting safety of converter transformer, which suffers severe stresses from both AC and DC electrical fields. Most insulation failures either in factory tests or in operation happened here. Based on experiences gained in a lot of UHVDC project design and test practices, focus on the shield electrode areas, the paper summarized 3 typical insulation structures, namely 'semi-covers''full covers' 'double-ring', provide a new type of 'Ω' shape-cover, all located in a multi-layer cylinder. The finite element method is used to calculate AC and DC electric fields and make safety assessment. The critical area of the structures and the improvement direction of field distribution and safety are theoretically pointed out. The calculation results and failure cases are matched and both indicate that: high DC field stress concentrate in the oil at the ends of 'semi-covers' where partial discharge is likely to occur and causing flashover along the inner surface of the multi-layer pressboard cylinders; the DC field stresses in the oil at the ends of 'full covers' are very high and the voltage gradient along the insulation surface of down-lead is also very high, which means high risk of flashover in oil or breakdown in paper; the non divided oblique oil gap from electrode to cylinder of 'double ring without covers' is rather large and has a high risk of discharge under AC; 'Ω'shape-cover may be a proper solution for both AC and DC insulation.