The present invention belongs to the nuclear power plant safety technical field and discloses a pipeline three-way area on-line fatigue life monitoring device and method. According to the pipeline three-way area on-line fatigue life monitoring device, a pipeline area pressure measuring device and a pipeline outer wall surface temperature measuring device are both connected with a server and input a measured pipeline system inner pressure value and a measured pipeline outer wall surface temperature value to the server; a pipeline inner wall temperature calculation module connected with the server can convert temperature into pipeline inner wall surface temperature; a temperature measurement data statistical analysis module performs classification and identification, so that a temperature range and number-of-times matrix table can be formed; and a monitoring point stress calculation module connected with the temperature measurement data statistical analysis module converts a matrix table type temperature statistical result transmitted by the temperature measurement data statistical analysis module into the stress value of a monitoring point and transmits the stress value of the monitoring point to a pipeline three-way area fatigue life evaluation system connected with the monitoring point stress calculation module. The pipeline three-way area on-line fatigue life monitoring device of the invention has the advantages of high precision, full range, high efficiency, on-line evaluation and the like.