As a basic industry, agriculture has a far-reaching impact on national economy. Based on input-output tables of China in 2005 and 2010, this paper gives a deep insight into the relationship between agriculture and national economy and its macroeconomic effects by the methods of input-output analysis, industrial correlation and macroeconomic effect analysis. The results show that the agricultural development is highly dependent on the national economy, but makes low contribution; agriculture contributes to promoting employment and improving the structure of national income distribution; despite the government's tax incentive, the agricultural equipments are updated slowly, and the corporate profits are at low level; agricultural export effect is far below the average industry level, but the import effect increases over the years, and final products are often used for domestic consumption. Finally, we set forth recommendations for improving the development of agriculture and national economy.