The invention discloses an over-temperature warning device for a power transmission line joint. The over-temperature warning device comprises a metal box body, a warning board and an easily-fused medium bar which can be fastened and connected with the power transmission line joint, wherein the warning board can slide up and down in the metal box body, lug bosses are respectively arranged at the left and right end of the upper part of the warning board, a slide-down outlet of the warning board is formed in the bottom of the metal box body, a limiting mechanism for preventing the lug bosses from sliding down is arranged at the bottom of the metal box body, the easily-fused medium bar transversely penetrates through holes in the warning board, and two ends of the easily-fused medium bar are bridged on two side walls of the metal box body. According to the over-temperature warning device for the power transmission line joint, the warning board slides down and stretches out from the metal box body when the temperature of the power transmission line joint is over high, so that an inspector can timely find failures and eliminate the failures as soon as possible, and the safety operation of a power transmission line is guaranteed.