The utility model relates to the technical field of power equipment state on-line monitoring. The utility model aims to provide an online monitoring device for a GIS equipment expansion joint. The problems that an existing telescopic joint is low in data accuracy and poor in real-time performance can be effectively solved. The device comprises a laser triangular reflection type displacement sensor, the laser triangular reflection type displacement sensor is fixedly mounted on the expansion joint flange through a mounting bracket; the output end of the power supply is connected with a signal transmission cable; the other end of the signal transmission cable is connected with a monitoring device in the signal line concentration cabinet; the monitoring device is connected with the control room through an optical fiber; the utility model has the beneficial effects that: the device is convenient to use; a laser displacement sensor is used for measurement. Measuring precision is extremely high, meanwhile, the displacement change situation does not need to be observed and recorded by personnel at any time or at regular time, the real-time performance is good, all the displacement change quantities are completely recorded and stored in the monitoring device, the displacement change numerical value at any time can be checked at any time, the labor intensity of workers is greatly relieved, and the GIS equipment operation and fault investigation efficiency is improved.