Abstract: In order to determine index weights reasonably on post-evaluation of water saving improvement in large-scale irrigation districts, thus scientifically evaluating the benefit of water saving improvement, index weights were calculated by combination weighting method based on unitization constraint conditions. And that method was applied to benefit evaluation of water saving improvement in Shaanxi Guanzhong irrigation districts. The results show the combination weighting method based on unitization constraint conditions can give consideration for both subjective and objective influencing factors, comparing with simple subjective weighting method or objective weighting method and other combination weighting methods. The combination weighting method is scientific and easy, and has better practicability.