The invention discloses a monitoring and alarming device for a departure from nucleate boiling ratio (DNBR) of a reactor core of a pressurized water reactor. Relevant factors which are detected by a detector is conducted through data preprocessing and a DNBR data base is obtained, and meanwhile, a DNBR nonlinear computational model which is trained by a cloud adaptive support vector machine is adopted to calculate a DNBR value, an output module displays the DNBR value, a differentiating signal is transmitted to a DNBR alarming controller to trigger alarming through a signal transmission device in a manual or intelligent mode, a DNBR alarming signal is fed back to the data base, a disaster signal which needs to be alarmed is transmitted to an alarming system, and relevant people conduct urgent elimination measurements on DNBR occurrence conditions inside the reactor core of the pressurized water reactor. The monitoring and alarming device for the DNBR has self-learning ability, continuously improves a DNBR computational model, and utilizing a central processing unit system to conduct real-time processing. The monitoring and alarming device for the DNBR is rapid in reaction speed, high in fault tolerance, capable of monitoring and forecasting changes of the DNBR inside the reactor core of the pressurized water reactor accurately and rapidly.