Watershed ecological water demand is the water quantity that the watershed ecosystem needs to maintain the health of ecosystem and to guarantee the ecological structure and ecological function.The research about watershed ecological water demand is important for the sustainable economic and social development.In the paper,ecological water demand and ecological benefits are calculated,analyzed and compared,especially the ecological benefits reduced by decreasing dry-off.Also the three themes are compared and analyzed.The results can provide some reference for making water allotment of the first phase of western line project of south-to-north water transfer.It is concluded that the integrated ecological benefits of 34.85 hundred million yuan will be brought with incoming water of 55×10⁸m³,the integrated ecological benefits of 47.18 hundred million yuan will be brought with incoming water of 122.8×10⁸m³,the integrated ecological benefits of 54.72 hundred million yuan will be brought with incoming water of 240×10⁸m³.