Due to limited resources of actors, third-party auditing has vital significance for ambient intelligence systems, which ensures the integrity of data stored in the information manager. However, Current public key based third-party auditing schemes are costly, while simple MAC based third-party auditing schemes can't fulfill two security requirements, namely the privacy-preserving and the integrity-against-TPA requirements. Taking both security and efficiency into account, this paper proposes a novel simple MAC based third-party auditing scheme, which is much more efficient than current public key based schemes, while still fulfills the above two security requirements. Therefore, it is feasible to be deployed in ambient intelligence systems.