This paper describes the changes of fishery environment focusing on the sediment by long-term monitoring in brackish lagoon area, central Vietnam. Acid-volatile-sulfide (AVS) and particle size distribution of sediment are measured at stated aquaculture period within 3 years. The AVS value of sediment became gradually high through the aquaculture season, which is the similar result as the previous report. However, after the aquaculture season experienced some flood events, reduction of the AVS value and the change of particle size distribution of sediment were found in several fishery sites. Also, some fishery sites where showed changes of particle size distribution of sediment through the year kept low AVS value. At some fishery sites faced environmental degradation that appeared as oxygen-deficient water. From the results, reduction of AVS value and the mechanism to keep low AVS value are proposed in 3 hypothetical models; [a] H₂S gas solved out from sediment into water with no particle size distribution change, [b] changes of particle size distribution of sediment under the strong disrupting effect by flood, [c] changes of sediment by moderate disrupting effect through the year. These results suggest that some fishery areas possibly have a mechanism of sediment recovery to keep low AVS value through the changes of sediment via annual flood. On the other hand, there is concern about risk and effect on fishery production that several fishery sites observed high AVS value and oxygen-deficient water in middle season of aquaculture, and also some sites assumed as hypothetical model [a] of the AVS value reduction.