To evaluate the influence of reservoir dick engineering on agricultural production and agricultural scale economy, this paper constructs a fixed effect model at first, in which Cobb-Douglas production function is chosen as the agricultural production function according to the real situation in China, and the influences of reservoir and embankment engineering, consumption of agricultural resources, scientific and technical level of agriculture and breeding, and the agricultural performance of reservoir and embankment engineering in different regions on agricultural output are measured effectively. Then, this paper employs the theory of scale economy to study the relationship between reservoir or embankment engineering and agricultural scale economy. The results show that the reservoir and embankment engineering have important effect on the growth of agricultural economy. In general, the contribution of embankment engineering to the growth of agricultural economy is more than that of reservoir engineering. With the improvement of reservoir and embankment engineering, the population benefited will increase gradually, as well as the total benefit and the benefit per capita, where the acceleration of benefit per capita is greater than that of the population benefited, which indicates that reservoir and embankment engineering are able to realize agricultural scale economy.