Bloom syndrome helicase (BLM) is an important member of RecQ family of DNA helicases. It participates in cell metabolism including DNA repair, recombination, transcription, telomere maintenance, and plays key roles in maintaining chromosome stability. The mutation of BLM helicase may lead to Bloom syndrome that characterized by genomic instability and a strong predisposition to many types of cancer. This study was studied the interaction of BLM helicase catalytic core (BLM^642～1290) with double-stranded DNA (dsDNA) by fluorescence polarization technology, and analyzed the related characteristic parameters to understand the DNA-binding and unwinding properties of BLM^642～1290 helicase. The results indicated that the binding and unwinding of the helicase and dsDNA was related to the length of 3'-tailed single-stranded DNA (ssDNA) in dsDNA substrates; the helicase preferred to bind the ssDNA tails of dsDNA substrates, and every helicase molecule may bind 9.6 nt ssDNA; the unwinding efficiency was the highest level when the 3'-tail ssDNA length of dsDNA substrates was 10nt, no longer increasing with the length. In addition, the blunt dsDNA was also bound and unwound by BLM^642～1290 helicase, but the binding affinity and the unwinding efficiency were smaller than 3'-tailed dsDNA. It may be concluded that BLM^642～1290 helicase was monomer in binding and unwinding to dsDNA substrates, and may unwind dsDNA in an inchworm manner. These results will provide the theoretical foundation for further studying the unwinding mechanism of BLM helicase.