53#, a strain of efficient alginate degrading bacteria, is isolated from rotted kelp using sodium alginate as the only carbon resource. Strain 53# is identified as Paenibacillus agaridevorans based on physiological characteristics and 16S rRNA sequencing. The optimaltion yield condition of strain 53# is identified at pH=8, T=25ºC, NaCl 15 g/L, and sodium alginate 15 g/L by orthogonal experiment and analysis, and the highest enzyme activity is 390.53±17.32 U/mL. Strain 53# has the advantages such as being cultivated easily, producing enzyme fast and having high enzyme activity. It can achieve a high efficiency for saccharification of alginate and thus has potential value to be utilized in the production of bioethanol from brown algae.