Farmland evapotranspiration generally refers to the sum of soil evaporation and vegetation transpiration. Development of the methods for farmland evapotranspiration estimation using thermal infrared remote sensing data is the challenging task in agricultural remote sensing. With more than three-decade development of space-borne sensors, the approaches to estimating evapotranspiration appear mature and have widely used in agricultural, meteorological and hydrological fields. This paper briefly describes the principles and the main types of current main retrieval algorithms of evapotranspiration, including the empirical method using the temperature difference between land surface temperature derived from thermal infrared remote sensing and air temperature, one source and two source models based on land surface energy balance, remotesensingbased Penman-Monteith equation, Priestley-Taylor parameterized by remote sensing based on using thermal infrared remote sensing data, etc. A number of algorithms have been proposed with a particular combination of multiple sources remotely sensed data. Importantly the unresolved problems of evapotranspiration estimation methods based on thermal infrared remote sensing in agricultural remote sensing are pointed out in the paper and the corresponding countermeasures are put forward.