The detector consists of a sensing cable, a signal processing unit and a terminal box. The sensor cable adopts a two-core cable structure, and its core wire is a metal wire with a diameter of 0.6mm. The metal wire is wrapped with two kinds of temperature-sensitive materials with different softening temperature points. The impedance between the core wires changes with the surrounding temperature. The signal processing unit is provided with a signal processing circuit, including signal acquisition, signal amplification and conversion circuit, display circuit, environmental temperature test circuit, etc. The signal processing unit is connected with a certain length of the sensing cable and the terminal box, and the signal processing unit continuously monitors the sensing cable and the temperature, and alarms the temperature rise, line break and short circuit caused by abnormal conditions.