The seabed-scour depth and the seabed elevation around the foundation of the offshore transmission tower are of great importance to the design of transmission towers and poles. Based on the Zhoushan long span power transmission project, this paper analyses natural scour according to measured submarine topography data, and researches on commonly scour and local scour around the foundation of the offshore transmission tower with a channel test under normal model. The study proves that the seabed condition is steady; the maximum amplitude of scour and silting for many years is 0.8 m, and the maximum local scour depth around the foundation of the offshore transmission tower is 7.5 m. On this basis, the total scour and the design elevation can be got, which provides reference for similar long span projects.