Reducing the transmission tower’s window dimension is important for lowering the overall project investment. The paper analyzes the factors that determine the transmission tower’s dimension and proposes a designing scheme of T-shaped insulator string in middle phase. The use of T-shaped insulator string has reduced the tower’s window dimension and weight, and decreased the electric field intensity and the transmission corridor width, subsequently improving the anti-lighting level and transmission capacity of transmission lines. So it is recommended to use the T-shaped string under better ground conditions to minimize the project investment.