Wheat relatives-derived alien genes provide most important resource to develop resistant varieties and protect wheat from the damage of powdery mildew. Pm57, a gene resided on the long arm of chromosome 2Ss#1 from Aegilops searsii caused broad-spectrum resistance against powdery mildew isolates at seedling and adult plant stage. In order to develop Pm57 mutants with loss of resistance to powdery mildew for further gene isolation using susceptible mutant-based resistant cloning approache, 10000 seeds of 89(5)69, a wheat-Ae. searsii recombinant stock containing Pm57 genes,were treated using 0.625% Ethyl methane sulfonate (EMS). M1seeds were then dense sowed in experiment station, and 1598 M2 lines were harvested. Of which 300 M2 lines randomly selected were used for resistance test by inoculation at seedling stage. After further validation of susceptible individual plants using two Pm57 specific molecular markers (X2L4g9P4/HaeIII and X284274) and 42 pairs of core primers,which are applied in DUS identification of wheat lines, 70 Pm57 mutants with loss of resistance to powdery mildew from 27 independent M2 lines were identified, with a mutant ratio as high as 9.0%. Thus, the newly-identified resistance-compromised mutants might lay a foundation for further cloning of the Pm57 gene.