Based on the ldquogrowth dragrdquo conceptual model of Romer, this paper computes the growth drag of water resource to agriculture through using panel data(1997~2006) of China's all provinces. The result shows the growth rate of agricultural production value fell 0.1121% per year on average because of the shortage of water resource. The number is seemingly small, but according to calculation, under the drag of water resource, the ratio will be reduced by 2.66% in 2030. Furthermore, it will be 3.74% in 2040 and 4.82% in 2050.