Based on the Solow model, technological progress and capital input are divided, and an improved Solow model is established. This model is applied to calculate the contribution rate of all input factors to economic growth and to evaluate quantitatively the social benefit of water-saving agriculture in China from 1991 to 2005. It is concluded that the contribution rates of all input factors are obviously different. Among these factors, the contribution rate of technological progress in water-saving agriculture is greater than the capital input of water-saving agriculture. Although periodic fluctuations are evident, the social benefits of water-saving agriculture generally have a stable development process. Water-saving agriculture is currently playing an important role in China.