Abstract: Heat transfer coefficient is one of the determinants in numerical simulation of thermal effluent. It is affected by the local water temperature, air temperature, pressure, humidity, wind speed, etc. As a result, few researches were done about heat transfer coefficient in the past. In this paper, one certain power plant in China is taken as an example to study heat transfer coefficient during thermal effluent. Results show that the change of maximum envelope area of high temperature rise (e.g. 4℃) in a full tide with water temperature is close to a linear distribution, while low temperature rise (e.g. 0.5℃) is close to an exponential distribution. At the same time, the maximum envelope area in a full tide will approach to a stable value when water temperature decreases. In other words, the influence of extreme low water temperature on thermal effluent is limited.