The Ningdong-Shandong ±660 kV HVDC transmission project is the first worldwide DC power transmission project in 660 kV voltage grade, where the single 12-pulse converter bridge is used resulting in the highest single valve volage and the largest valve tower dimension. Therefore, it is difficult for valve testing. In this paper, the items and parameters for the insulation test of valves are introduced, and the difficulties encountered during the test as well as the coutermeasures employed are illustrated.