In the traditional protection system of generation II reactor, over-temperature protection signal is usually used. This protection method is conservative and indirect, however, the Departure from Nucleate Boiling Ratio (DNBR) on-line monitoring system can directly monitor the changes of safety parameters. In order to directly monitor the safety margin of Hualong Pressurized Reactor (HPR1000) and improve the flexibility of reactor safety operation, this paper puts forward a DNBR on-line monitoring calculation model with both calculation speed and solution accuracy based on the basic model of thermal design of PWR core, and develops a set of DNBR on-line monitoring system for HPR1000. The calculation model of on-line monitoring system is fully verified from three aspects: independent verification of core, verification of analog signal and verification of actual operation data of HPR1000. The results demonstrate that the precision of the on-line monitoring system is very high, and equvalent with the sub-channel code. In conclusion, the DNBR on-line monitoring system could satisfy the needs of HPR1000 engineering design. © 2024 Atomic Energy Press. All rights reserved.