The invention relates to an electromagnetic shielding shell for a power transmission line one-line monitoring device. The electromagnetic shielding shell consists of an outer metal shell, an inner metal shell and a horizontal metal casing pipe, wherein the outer metal shell is shaped like a horizontal cylindrical tank; the metal casing pipe is sleeved on a high-voltage line and passes through the outer metal shell; the inner metal shell is positioned in the outer metal shell; and the power transmission line one-line monitoring device is arranged in the inner metal shell. The electromagnetic shielding shell has the advantages of small volume, low cost, convenience for mounting and the like, has a good shielding effect, and can guarantee normal operation of the power transmission line one-line monitoring device.