Abstract: Objective: This study aimed to screen a strain with high alginate lyase activity, optimize the fermentation conditions, and evaluate the degradation effect on Sargassum of the strain. Method: The high alginate lyase producing strain was screened by using sodium alginate as the sole carbon source on agar medium and alginate lyase activity from Sargassum and Enteromorpha samples. The strain was identified based on morphological, physiological and biochemical characteristics, 16S rDNA sequence analysis, and phylogenetic tree construction. The fermentation conditions for enzyme production were optimized through single factor experiments and response surface methodology. The sargassum-degrading effect of the strain was evaluated by the the determination of reducing sugar and scanning electron microscopy. Results: The results indicated that a high alginate lyase producing strain ML070 was obtained by screening and identified as Exiguobacterium mexicanum. The optimal fermentation conditions for alginate lyase production were sodium alginate 1.1%, yeast powder 0.34%, NaCl 2.61%, initial pH9, inoculum size 2%, agitation speed 180 r/min, fermentation temperature 25 ℃ and fermentation time 48 h. Under the optimized fermentation conditions, the alginate lyase activity reached 254.961 U/mL , which was 1.79 times compared with initial activity. The alginate lyase with extensive substrate specificity and the products of alginate hydrolyzed by alginate lyase were determined as alginate oligosaccharides with polymerization degree 2 and 3, indication the alginate lyase was an endo-type enzyme. When Sargassum cultured with the strain for 72 h, the Sargassum cell morphology and surface structure were obviously damaged. Conclusions: The Exiguobacterium mexicanum MAL070 with good degradation ability of Sargassum and laid the foundation for potential application of Sargassum.