[Objective] This study is aimed to isolate and identify a strain which can produce dextranase, characterize the enzyme properties and study its application in Streptococcus mutans dental biofilm. [Methods] The strains derived from marine environment were screened by observing transparent circle on agar plates. The strains were identified by morphology, physiological characteristics, and phylogenetic analysis based on 16S rDNA sequences. The inhibitory effect of the dextranase on Streptococcus mutans dental biofilm was assessed with biofilm models in vitro. [Results] A dextranase-producing strain KQ11 was isolated from sea mud and preliminarily identified as Arthrobacter sp.. The optimal growth conditions of the strain were 30 °C, pH 7.5, and NaCl concentration of 0.4%. The optimal catalytic conditions of the enzyme were 45 °C, pH 5.5. The dextranase effectively prevented the dental biofilm formation by Streptococcus mutans. [Conclusion] The dextranase from the strain KQ11 can effectively degrade Streptococcus mutans dental biofilm, and thus it can be added to the oral care products such as mouthwash.