Abstract: Promoting some key industries, such as agriculture, became an important way to reduce poverty and to maintain a sustainable economic development. Agricultural industry built a bridge between public policy and poverty alleviation and showed some significant mediating effects: public policy promoting the process of poverty reduction effectively through agricultural development. By establishing a state space model, which incorporates three evaluating index systems, including public policy, agricultural development, and poverty alleviation, this paper evaluated the effects and trends of the poverty alleviation through agricultural development at different stages from 2000 to 2015 and also verified the mediating effects of agricultural development in the process of poverty alleviation. Results show that 1) the mediating effects of agricultural development on poverty alleviation is very significant, which contributes about 45.25%; 2) the impacts of public policy on poverty alleviation and agricultural showed a downward trend, but the effect of agricultural development on rural poverty alleviation is increasing year by year; and 3) the diversification of poverty reduction system has been basically formed in China. In addition, this research also found that the endogenous poverty reduction mechanism has been gradually improved and has been working effectively. The promotion of public policy on agricultural and poverty reduction has in the trend of diminishing marginal effects. To enhance the agricultural role of poverty alleviation, this paper suggests to strengthen the support of agriculture, especially the agricultural products with unique characteristics, and to shift the strategy of poverty alleviation to endogenous development mode.