Combined with the discussion of key scientific problems concerning ecological security and water requirement in arid region, the paper puts forward a fact that Arid inland river basin, hydrologically, represents a relative closed catchments and a self-existent hydro-ecosystem, in which the water and eco-environment are interrelated and controlled, and formed the main body of river ecosystem. The ecological security analysis on fragile ecosystems in arid inland river basin are centering on water processes. Water processes control the ecological processes and affect the stability of ecosystems directly. Water resources development and utilization and water cycle have an important impact on the function of ecosystems. The study of reasonable groundwater table for the restoration and growth of natural vegetation is the basis for the establishment of ecological water requirement, and the study of water requirement has a very important significant in both ecological security and water use in arid land. By demonstration analysis, the present ecological water requirement maintaining the ecological security in the Tarim River was estimated at 31.74×10⁸m³, of which 9.95×10⁸m³, 18.47×10⁸m³ and 3.32×10⁸m³ for the upper, middle and lower reaches, respectively.