The invention provides a calculating method and a calculating system for daily evapotranspiration of a mixed pixel in a remote sensing image. The calculating method comprises the following steps: S1,acquiring evaporation ratio of each mixed pixel according to area ratio of a sub pixel, corresponding to the same land type in the each mixed pixel of the remote sensing image, and the whole mixed pixel, and evaporation ratio of the sub pixel corresponding to the same land type in the each mixed pixel; S2, acquiring the daily evapotranspiration of the mixed pixel according to the evaporation ratioof the each mixed pixel and daily cumulated effective energy. According to the calculating method and the calculating system provided by the invention, contributions of different land types in the mixed pixel to evapotranspiration are combined to solve the evaporation ratio of the whole mixed pixel, and the evaporation ratio of the mixed pixel is multiplied by the daily cumulated effective energy, so that extension of remote sensing instantaneous evaporation in time scale is realized, and the daily evapotranspiration of the remote sensing mixed pixel is solved. The accuracy of a remote sensing technology for non-uniform estimation of daily evapotranspiration of earth surface is improved, growth and development of plants and atmospheric circulation can be better analyzed, and management ofregional water resources is facilitated.