The Aha Lake, as a seasonally oxygen-absent man-made reservoir, has been polluted by acidic mining drainage and domestic sewages for a long time, with iron, manganese and sulfate excessively enriched in water and sediment. By means of microbe counting, the analysis of trace metals in pore water and electronic acceptors for organic matter decomposing, we have found that strong biogeochemical remobilization of trace metals occurred near the water-sediment interface. The microbial reduction of iron, manganese and sulfate took place in different parts throughout the sediment core with the extend of iron reduction lower than that of sulfate reduction, which happened in the surficial sediments and hampered the upward release of some trace metals to some extent. Some trace metals in pore water, due to the “dual releasing” effects caused by the reduction of Fe³⁺ and Mn⁴⁺ at varying depth, show a tendency of being enriched excessively in the upper 10 cm of sediment. In this study, we discussed the microbiological mechanism of trace metals enrichment in surficial sediments and the environmental condition, with an attempt to realize the unsteady mobilization of trace metals and their potential harm to overlying lake water in the Aha Lake, Guiyang.