The DIF-free-then-DIF (DFTD) strategy consists of two steps: (a) select a set of items that are the most likely to be DIF-free and (b) assess the other items for DIF (differential item functioning) using the designated items as anchors. The rank-based method together with the computer software IRTLRDIF can select a set of DIF-free polytomous items very accurately, but it loses accuracy when tests contain many DIF items. To resolve this problem, the authors developed a new method by adding a scale purification procedure to the rank-based method and conducted two simulation studies to evaluate its performances on DIF assessment. It was found that the new method outperformed the rank-based method in identifying DIF-free items, especially when the tests contained many DIF items. In addition, the new method, combined with the DFTD strategy, yielded a well-controlled Type I error rate and a high power rate of DIF detection. In contrast, conventional DIF assessment methods yielded an inflated Type I error rate and a deflated power rate when the tests contained many DIF items favoring the same group. In conclusion, the simulation results support the new method and the DFTD strategy in DIF assessment.