For reducing the steric hindrance and nonspecific binding of the target DNA, the dextran was used as molecular arms to be immobilized on the surface of magnetic nanoparticles (MNPs). Magnetic separation was used in preparation of dextran-MNPs (DMNPs). Aspartic acid and aminated DNA probe were successively modified on the dextran immobilized on the surface of MNPs. These probe-DMNPs were successfully applied to detect biotin-labeled PCR product of E. coli O157:H7 genome by hybridization. Then the complexes were bonded with streptavidin-modified alkaline phosphatase (ALP-SA). Finally the chemiluminescent signals were detected by adding 3-(2-spiroadamantane)-4-methoxy-4- (3-phosphoryloxy) phenyl-1,2-dioxetane (AMPPD). The results showed that this method had a good specificity, and higher sensitivity than that when only MNPs were used as solid carriers.