[Background] Microbial proteases have broad application prospects in industrial biotechnology. Among microbial proteases, alkaline proteases account for more than 50% of the world's total enzyme production. It is of great significance to explore new microbial resources that synthesize alkaline proteases. [Objective] Strains with high production of alkaline proteases were screened from the sea mud of Hainan offshore shellfish breeding base and their growth characteristics were explored. Moreover, the conditions for the enzyme production were optimized. Thereby, new protease-producing resources were obtained. [Methods] Casein medium was employed for screening the protease producers and the yielded strain was identified by morphological and phylogenetic analysis. The enzyme production conditions of the strain were optimized by response surface methodology. [Results] Strain F3 with high production of alkaline protease was screened out and identified as Serratia marcescens. The enzyme activity in the fermentation broth of the strain was up to (339.36±4.30) U/mL under the optimal conditions. [Conclusion]S. marcescens F3 has strong ability to produce alkaline protease.