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. (Contains 4 tables and 2 figures.)