Abstract: A method for reckoning the environmental background water temperature and calculating the temperature rise in a temperature rise area of a newly-built coastal power plant is provided. The reckoning method includes: S01, before an operation of the newly-built coastal power plant, building a water temperature reference station; S02, continuously observing and recording water temperature data of the water temperature reference station and other water temperature observation stations; S03, analyzing the water temperature data obtained in S02; and S04, reckoning a sea surface natural water temperature of the other water temperature observation stations.