Highly sensitive and specific detection of miRNA still remains challenging. In this work, a simple and sensitive fluorescence biosensor has been developed for detection of miRNA by combining the magnetic nanoparticles coated with poly-dopamine (MNPs@PDA) with duplex specific nuclease (DSN). The MNPs@PDA could be easily synthesized via autoxidation of dopamine on the surface of magnetic nanoparticles. The MNPs@PDA could specifically bind with the FAM-labeled single-strand DNA (ssDNA) probes via polyvalent metal-mediated coordination, resulting in the quench of fluorescence signal. The MNPs@PDA exhibited good anti-interference performance in complex matrix. The inner filter effect (IFE) of the MNPs@PDA could be eliminated via magnetic separation. In the presence of specific miRNA, DSN digested the DNA in the DNA–miRNA duplexes and small fragments were formed. The force between these small fragments and MNPs@PDA was negligible, resulting in the occurrence of fluorescence signal. Due to the incorporation of DSN, signal amplification was realized via the recycling process. The established method achieved a low detection limit down to 0.42 pM. The linear concentration range was from 5 pM to 5 nM. Moreover, this method also had high specificity. Remarkably, the target miRNAs extracted from human cells were detected by using the sensing platform.