Many researches focus on the ecological water demand accounting problem. But it has a certain restriction of several potential evapotranspiration models commonly used in China and abroad. Combined with the northeast area characteristic, the paper has chosen Harbin as the example. It has selected a series time of indicator data and compared the applicability and disadvantage about these models. The paper proposes the method to amend and optimize the Hargreaves (HM) model. It also revised the parameters. The correlation coefficient of amending HM and the PM calculations is 0.826, and the relative mean squared deviation is 0.119. It has a good correlation of calculating the evapotranspiration calculation of Harbin. And it will lay the foundation for the quantitative accounting ecological water demand of the northeast region.