The invention discloses a ring main unit cable head temperature on-line monitoring device and a temperature measuring method, belongs to the technical field of electric power and particularly relates to a cable head temperature measurement device. A, B and C three-phase digital temperature sensors are installed on a wire incoming cable head and a wire outgoing cable head of a ring main unit and serve as temperature acquisition modules; a 2.4 GHz high-performance radio frequency transceiver with the Zigbee wireless transmission function is installed on a front side panel of the ring main unit and serves as a wireless transceiver; a single-chip microcomputer is arranged in the ring main unit and serves as a control module; the control module single-chip microcomputer is connected with a DTU through an RS485 interface and a signal line and then connected with a main station front-end processor through a communication transmission module, and the main station front-end processor is used for presenting temperature information of the cable heads. Due to the ring main unit cable head temperature on-line monitoring device, the temperature of the cable heads in the ring main unit can be accurately measured and monitored online, and a detection error value meets the requirement of a target value.