The middle reaches of Yarlung Zangbo River (YR) and its two tributaries (Lhasa River and Nianchu River) is a main agricultural region in central Tibet Autonomous Region. Drought monitoring has significant meaning for local crop growth monitoring, crop yield assessment and disaster monitoring. It also has great theoretical importance for understanding local energy and water balance status. In this paper, MODIS data on 14 April 2003, 16 October 2003 under nearly clear weather conditions are selected as the spring and autumn cases. The land surface parameters, such as land surface temperature (LST), albedo, Normalized Difference Vegetation Index (NDVI), emissivity etc., have been derived from different algorithms for MODIS data. In combination with meteorological data, the drought severity index (DSI) is determined by applying Surface Energy Balance System (SEBS). The estimations show that spring drought is much more severe than autumn drought. The river runoff is the dominant factor for spring drought. The spatial variations of drought conditions in the middle reaches of YR and its two tributaries can also be clearly identified.