Necessity of establishing the early-warning mechanism of food security is introduced, which is mainly shown in five aspects. First is the particularity of China's national and food condition: China has large population and immense consumption base. Once food insecurity occurs in an all-round way, China could not buy all the food needed from the international market. The second aspect is the importance of food as a commodity: food is the most basic means of subsistence for human being, playing a role of social "stabilizer". It is also the basis of the national economy. The third aspect is the fluctuation of food supply: inter-annual variation of grain production is significant in China; and early-warning information of food security is needed in order to stabilize the fluctuation of food supply. The fourth one is the potential crisis in food marketization: both supply and demand of food are relatively small, which is unable to make great response to market price. Thus, risk of food security is exacerbated. The fifth aspect is the political nature of food as a commodity: food, an important strategic resource, has important impact on the political and military actions of a state. Measures to improve the early-warning mechanism of food security are discussed. Firstly, set up an independent early-warning mechanism, which is the premise to improve the early-warning mechanism of food security. Secondly, establish the operating principle of early-warning mechanism, mainly including systematic principle, qualitative and quantitative principle, principle of high efficiency, and principle of integrating legal, administrative and economic means. Thirdly, ascertain the operation mode of early-warning mechanism. Government leading mode should be selected based on the actual situation of China. Fourthly, increase the input in early warning and improve the quality of early-warning information of food security. Fifthly, confirm the legal responsibilities of related subjects, in order to ensure the normal operation of early-warning mechanism and to exert its due social function.