A method based on Penman-Monteith equation recommended by FAO is established in this paper to calculate total reference evaporation of year through daily temperature. After being validated, the method is used to get the change of Yangtze River's reference evaporation under increasing temperature. The study shows:Reference year-evaporation will enhance together with temperature. The relative enhancement in upper reaches of the Yangtze river is bigger than in middle and lower reaches of the Yangtze. The relative enhancement is similar in different month. The distributing of temperature enhancement in a year shouldn't be omitted and the result under this circs is more accurate. The relative enhancement of reference evaporation by 1℃ is about 5% in upper reaches of the Yangtze river and 4% in middle and lower reaches.