Only for the Public interests, the land can be expropriated .It is the common view about the policy of the land expropriation in the worldwide. However，because of the lack of the definition on the public interests，many social problems appear during the judicature practice. The definition of the public interests is the first important issue that should be settled during the process of the land expropriation. This paper will analyze in detail the theoretical definition of public interest, its' existing defect and improvement. Lastly, the writer gives the honest opinions about how to perfect the system of land expropriation as defined in the public interest.