The invention discloses epitope for an NY-ESO-1 tumor antigen. The amino acid sequence of the mimic epitope is Ser-Leu-Leu- Met-Phe-Ile-Thr-Trp-Cys, namely SLLMFITWC; the mimic epitope can raise a CTL immunity response, can undergo a cross reaction with a natural epitope, has the characteristics of strong immunogenicity, immune tolerance breaking, strong specificity, safety, low cost, and easy synthesis and storage, and can be used to prepare tumor-therapeutic polypeptide vaccine. The invention also discloses a method for predicting the mimic epitope, which comprises steps of the establishment of a structural model of a TCR-pMHC complex, the analysis of TCR binding sites of the epitope, and the analysis of amino acid replacement of the TCR binding sites of the epitope. The method is also applicable to the computer-aided modification of CTL epitopes of other antigens, and can provide a useful tool for the design and research of therapeutic polypeptide vaccine.