[Objective] To screen an alginate-degrading strain and to improve the activity of its alginate lyase by optimizing the fermentation conditions.[Methods] A strain capable of degrading alginate was screened by the steps of enrichment culture, screening, and re-screening by using seawater and sea mud collected from the sea area of Zhangzhou (Fujian, China). Then, the strain was identified according to its 16S rRNA sequence analysis, physiological and biochemical characteristics, mycelium morphology and colony characteristics. The conditions of enzyme production of the strain were optimized by single factor and orthogonal test. [Results] The strain, belonging to the genus Cobetia marina, was named Cobetia marina HQZ08. The optimal medium of the strain was composed of 7.00 g/L sodium alginate, 3.00 g/L peptone, 30.00 g/L NaCl, 1.25 g/L K₂HPO₄·3H₂O. The optimal fermentation conditions were as follows:the optimum inoculum was 2%, the inoculation age was 12 h, the initial pH of the culture medium was 7.0, the culture temperature was 25℃, and the incubation time was 24 h. The enzymatic activity of alginate lyase reached 68.5 U/mL after optimization. Enzymatic hydrolysates were determined to be alginate oligosaccharides. [Conclusion] HQZ08 strain can be used to degrade alginate to produce alginate oligosaccharides with a degree of polymerization of 2-6.