Geochemical characteristic of sulfur and iron was investigated in three types of sediments including algae accumulated sediment, dredged sediment and sediment away from lake shore. It was found that iron reduction dominated in all sediments rather than sulfate reduction. Both reduction concentrated in 0-15 cm surface sediments according to the distribution of microorganisms of iron and sulfur species. Accumulation of dS^2- and acid volatile sulfide(AVS)was not observed in sediment away from lake shore, indicating that sulfate reduction is minor. Iron reduction was more active than sulfate reduction as the mean concentration of dFe²⁺ was 5.2 folds of dS^2-. Sulfate reducing bacteria (SRB) abundance and dS^2- content in dredged sediments were the lowest and dFe²⁺ release was not found, suggesting the depressed iron and sulfate reduction by dredging. SRB abundance in algae accumulated sediment reached to 2.7% and intensive dS^2- release and AVS deposition were found, indicating the strong sulfate reduction. Active iron reduction was also found with high dFe²⁺ concentration in algae accumulated sediment. However, the mean abundance of iron reducing bacteria (FeRB) was only 0.6%, which was similar to other sediments and was obviously not able to explain the huge difference of dFe²⁺ concentrations. Thus, microbial iron reduction was not the main pathway of iron reduction. Considering the corresponding dissolution of Fe(Ⅲ) oxides and deposition of iron sulfides, sulfide-mediated chemical iron reduction was the dominating pathway. The mean concentration of dFe²⁺ was 4.8 folds of dS^2-, and iron reduction rate was 7.4 folds of sulfate reduction rate by monitoring dS^2- and dFe²⁺ variations, suggesting iron reduction was more active than sulfate reduction. However, as AVS generation could consume dS^2-, the real rate of sulfate reduction was higher than the measured rate. Transition of iron and sulfur cycling would cause various negative effects and even extreme cases including "dead zones" in marine and "black bloom" in freshwater lakes. Although similar phenomenon was only observed in algae accumulated sediment, concern should still be raised.