Mixed pixels constitutre AVHRR image. Evapotranspiration （ET） calculated based on AVHRR image certainly includes some errorous information of ground objects which have no meaning as far as ET is concerned. Registration of AVHRR image and high spatial resolution image is carried out. A meteorology-RS combination model is used to calculate the ET of the AVHRR image and that of the cropping area of the corresponding high resolution image respectively. The difference of these two ETs is given. The error is analysed and a corresponding statistical calibration method adaptable to the test region is given.