The invention provides a blocking method of a U-shaped net. Each U-shaped network comprises a side surface first netting rope, a side surface second netting rope, a bottom surface first netting rope, and a bottom surface second netting rope, the U-shaped network is built on a supporting rope, and the supporting rope comprises an upper cross-arm first support rope, an upper cross-arm second support rope, a net edge first support rope, a net edge second support rope, and a middle support rope. The method includes: a circulation rope is employed to pull the upper cross-arm first support rope and the upper cross-arm second support rope; the circulation rope is employed to pull the net edge first support rope, the net edge second support rope, and the middle support rope; the side surface first netting rope is arranged between the upper cross-arm first support rope and the net edge first support rope; and the side surface second netting rope is arranged between the upper cross-arm second support rope and the net edge second support rope; and the bottom surface first netting rope is arranged between the net edge first support rope and the middle support rope; and the bottom surface second netting rope is arranged between the net edge second support rope and the middle support rope. According to the method, important facilities below transmission lines can be protected from being damaged by the transmission lines or pulling ropes.