Abstract: Frequent summer typhoon in subtropical regions brings significant impacts on water environment in estuaries. Based on the in-situ hydrographic observation in 2014, this paper portrays the characteristics and changes of water H-O isotope compositions in the Nandu River Estuary during the typhoon Kalmaegi. Results show that atmospheric precipitation, which is mainly derived from the water vapor transported from the tropical Pacific, is the main factor controlling the compositions of the H-O isotope in the typhoon period. After the typhoon, evaporation is strengthened with the decrease in rainfall and increase in temperature, and as the results, the contents of H-O isotopes are increased in water. The large amount of fresh water brought in by the Typhoon Kalmaegi will certainly bring in changes in the estuarine environment. The tidal current limit will move towards the river mouth, water mix strongly, and brackish water front form outside the river mouth. This study provides observational evidence for understanding the impact of typhoon on the water cycle of small-medium rivers in subtropical regions.