The invention discloses an online temperature detection method for a three-phase box-sharing GIS (Gas Insulated Switchgear) bus joint. The online temperature detection method comprises the following steps: determining a temperature rise most sensitive point corresponding to the three-phase bus joint on the surface of a shell of a GIS bus; acquiring the radiuses of a conducting rod and the shell of the GIS bus, load current, ambient temperature and the three-phase temperature of the temperature rise most sensitive point; acquiring corresponding function relations among the joint temperature of an equipment bus, the radiuses of the conducting rod and the shell of the equipment bus, the load current, the ambient temperature and the three-phase temperature of the temperature rise most sensitive point; detecting the joint temperature of the three-phase bus joint. By adopting the online temperature detection method, the temperature of the GIS bus joint can be detected accurately on line without cutting off, and detection results are not influenced by the external environmental factors. The equipment structure does not need to be changed, and high safety and high practicability are realized. Moreover, the temperatures of a plurality of GIS bus joints in a transformer substation can be detected at the same time, and convenience is brought to detection automation.