According to the data of the second national land survey, the farmland area of China at the end of 2009 was 200 million mu more than the original statistical data. This raises widespread concern. Analysis is carried out from the perspective of task, technique, necessities and effect on China’s grain security of the second national land survey. Through analysis, it can be deemed that data of the second national land survey are reliable. It overcomes defects of traditional survey projects and will be of profound significance for grasping current situations of farmland resources in China. Farmers are grain production entities, while the grain yield survey of farmers is calculated through sampling survey of per unit area yield and the sown area. Therefore, the increase in farmland area may indicate underestimation of the grain yield, and the grain security risk of China will decline accordingly.