The invention discloses an application of a tea polypeptide with an amino acid sequence being AGFAGDDAPR, namely, an application of the tea polypeptide or a precursor peptide of the tea polypeptide in preparation of a drug or a healthcare product for treating and/or preventing type-2 diabetes mellitus. The tea polypeptide has a remarkable inhibition effect on DPP-IV (dipeptidyl peptidase-IV) activity, thereby having good potential in preparation of the drug or the healthcare product resistant to type-2 diabetes mellitus and preparation of a DPP-IV inhibitor. The invention further discloses pharmaceutical composition for treating type-2 diabetes mellitus. The pharmaceutical composition contains an effective dose of the tea polypeptide represented as SEQ ID NO.1, the precursor peptide of the tea polypeptide and a pharmaceutically acceptable carrier.