The invention provides a device for online monitoring and controlling thermal stress of components of a steam turbine and a method, belonging to the technical field of the steam turbine. The invention is characterized in that the device consists of a compute server, a database server, a web server, a user browser and a plant supervisory information device (SIS device); the web server is connected with the user browser, the database server and the compute server respectively; the compute server is connected with the database server which is connected with a digital electric-hydraulic control device (DEH) of the steam turbine by the plant supervisory information device (SIS device). The invention has the advantages of providing the device for online monitoring and controlling thermal stress of a plurality of durable components of the steam turbine and realizing online computing and controlling of thermal stress of a plurality of durable components of the steam turbine.