The invention relates to the technical field of nuclear power fatigue life monitoring, and in particular discloses a device and a method for carrying out online monitoring on the fatigue life of a steam generator. The monitoring device comprises a fatigue monitoring point temperature measuring device and a fatigue monitoring point pressure testing device which are respectively connected with a server and transmit temperature data of a metal wall surface and fluid as well as primary lateral pressure and secondary lateral pressure of the steam generator to the server, and the server is used for processing the data and displaying monitoring results; a fatigue monitoring point stress processing module connected with the server converts the temperature and pressure parameters, which are measured at a monitoring point, into monitoring point stress calculation results by using a stress calculation function, and transmits same to a monitoring point fatigue life online analysis module connected with the fatigue monitoring point stress processing module for converting the results into a cumulative fatigue damage coefficient. The device and the method have the advantages of being high in accuracy and high in efficiency, realizing online evaluation, and the like.