Although water has the central function of the bloodstream in the biosphere especially in arid or semi-arid regions such as Yanhe watershed in northwestern China, yet the very limited attention is paid to the role of the water-related processes in ecosystem. In this research, based on continuous 30-year weather condition report data and 10-year soil water content data in Yanhe watershed, a comparatively accurate method for measuring the minimum, moderate and maximum ecological water requirement of plant-soil landscapes has been put forward by using experiential models and GIS spatial analysis. Additionally, the change characters of ecological water requirement have been addressed from spatial-temporal dimensions. The results show that: (1) In Yanhe watershed, the ecological water requirement of plant-soil landscapes is more in summer than in other seasons; (2) The landscapes of ecological water presenting highest requirement are forest-cohesive loam, forest-loam and forest-sandy loam, while the landscapes of ecological water presenting lowest requirement are grass-sandy loam, grass-loam and grass-cohesive loam; (3) The mean annual precipitation in Yanhe watershed can only satisfy the minimum ecological water requirement, but cannot meet the moderate and maximum ecological water requirement.