The invention discloses a protection device of an overhead power transmission line. The protection device comprises a support assembly, wherein the support assembly comprises a vertical plate and a horizontal plate, the horizontal plate is arranged at an upper end of the vertical plate, a left end surface of the horizontal plate is fixedly connected with an upper end of the vertical plate, an airspeed measurement device is arranged on the support assembly, a wind deviation prevention device is arranged below the horizontal plate, a power transmission line protection assembly is connected to apart below the wind deviation prevention device and comprises an outer frame, the outer frame is connected with the wind deviation prevention device, a power transmission line sleeve is arranged at amiddle part of the outer frame, a power transmission line is arranged at a middle part of the power transmission line sleeve in a sliding way, four support rods are symmetrically arranged at a circumferential center of an outer surface of the power transmission line sleeve, power transmission line insulators are arranged on the support rods, and telescopic assemblies are symmetrically arranged onthe vertical plate at a position opposite to the outer frame in a leftward-rightward way. By the protection device, the situation that the power transmission line is easy to break during alternate cooling and heating if the power transmission line between two wire rods is excessively horizontal during laying process of the overhead power transmission line is solved, and the problem that the powertransmission line also can be broken if wind is two large during wind blowing is prevented.