[Objective] The the responses of soil microbial biomass and enzyme activities to a flooding gradient in mangroves were determined to reveal the dynamics of soil quality and its influencing factors with increasing flooding period in order to provide a theoretical basis for mangrove protection and management. [Methods] We selected a mangrove plantation located in Fucheng town on the Leizhou peninsula, Guangdong Province, as the study site. We collected surface sediments (0-20 cm) from low, mid-low, mid, mid-high, and high-intertidal zones according to the distribution of intertidal zones and the type of mangrove communities. Soil microbial biomass carbon (MBC), nitrogen (MBN), phosphorus (MBP), and enzyme activities of different intertidal zones were measured and analyzed. [Results] As flooding frequency increased, MBC and MBN significantly decreased by 96.7% and 98.4%, respectively. The highest values were observed in the high-intertidal zone. The MBP fluctuated and decreased, with the highest value observed in the mid-high-intertidal zone. Microbial entropy carbon (q_MBC) and nitrogen (q_MBN) initially increased and then decreased with time, while the microbial entropy phosphorus (q_MBP) fluctuated with a decreasing trend. The highest q_MBC, q_MBN, and q_MBP values were observed in mid-high-intertidal zone. The activities of β-glucosidase and acid phosphatase initially rose and then declined, with the highest values observed in the mid-high-intertidal zone. The activities of urease significantly decreased by 20.2%, and the highest value was observed in the high-intertidal zone. Redundancy analysis indicated that q_MBN, MBN, and MBC were the main factors influencing soil enzyme activities in the surface soil, accounting for 50.9% of the total variation in enzyme activities. [Conclusion] Flooding gradient had a significant effect on soil microbial biomass and enzyme activities, and soil fertility decreased substantially with increased flooding time. An increase in nitrogen fertilization may improve soil quality and promote mangrove afforestation and growth.