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 number of series-connected thyristors. Based on the H400 valve developed by China Electric Power Research Institute (CEPRI) together with AREVA company of France, the valves applied in the project are optimized by introducing valve section capacitor and equipping with new shielding case verified through type test. Furthermore, the electrical design achievements of the thyristor valve in the project is reviewed.