Acid-volatile sulfides(AVS), which can form insoluble sulfides complexes with minimal biological availability with a number of cationic metals, is a key partitioning phase controlling metal activity in the sediments. The ratio of simultaneous extracted metal(SEM)to AVS is promising to predict metal toxicity. It was found that the sediments with SEM/AVS≤1 were of no metal toxicity and may be toxic with SEM/AVS 1. The authors investigated the seasonal and vertical distribution of AVS and SEM in the sediments cores of Lake Donghu. It was found that AVS is higher in summer than that in winter. It increases early and then decrease during the top 20cm layer of the sediments. This variation in AVS levels is attributed to temperature and sulfide oxidation in the suboxic top layer and sulfate reduction in the underlaying anoxic sediment layer. Comparison with AVS, SEM vertical distribution is not obvious. Thus, the SEM/AVS ratio first drops from 1 to less than 1, and then increases at greater depthes, even higher than one in same case. The results indicate that SEM/AVS ratios in mixed homogenized and single seasonal samples are generally not suitable for the assessment of potential metal toxicity of sediments.