After the Three Gorges Dam impoundment, the original water-level fluctuation rhythm of Yangtze River change dramatically. As a result, two new kinds of drawdown zones appear in Three Gorges Reservoir (TGR): the preupland drawdown zone (PU-DZ) which never submerged before impoundment and the preriparian drawdown zone (PR-DZ) which is the former riparian zone of Yangtze River. The water-level fluctuation rhythm of PU-DZ is high in winter because of impoundment, and low in the summer because of flood discharged. Meanwhile, the PR-DZ is affected by both the impoundment in winter and the flood in summer, so the water-level can raise high in winter and summer. Apparently,they both have differences with the water-level fluctuation rhythm of natural riparian zone (NRZ), which is only high in summer for the flood. Annuals can be divided into three types according to their germination and fructified characteristics: annuals germinating in winter fruitifying in summer(AGWFS), annuals germinating in spring fruitifying in autunm (AGSFA) and the common annuals. In this study we chose five sample sites in each kind of habitat and survey them before the freshet and after the impoundment subsiding by fixed quadrats in different transects. All these three types annuals can be discovered in each drawdown zone, however, the proportion of the number of different type's species and their population size are significantly different. The major reason is discrepancy of occurred submergence time in different zones. There are 73 kinds of annuals found in NRZ. The number of AGWFS is 45, its proportion of species number are 52.26-67.46 as the elevations decrease, which is obviously dominant compared with the other two types, because they can avoid the disturbance of summer flood. The total number of annual in PU-DZ is 85, wherein the number of type of AGSFA is 45. The proportions of this type in the total species number are 59.42-82.44 as the elevations decrease. They are dominance in PU-DZ, for the water-level is high in winter from November to the next year's January has little effect on them. However, AGWFS can hardly germinate in this stressful environment. AGWFS are more easily seen in high elevation because this area can end of the impoundment. In PR-DZ, through the total number of annuals is 62, which is not significantly lower than the other two regions. Summer flood and the winter impoundment disturb so frequently that any type of annual cannot get dominance in this environment, the proportions of AGWFS in each elevation are 30-52.57. Meanwhile the proportions of AGSFA in each elevation are 13.33-48.32. Neither of them can get dominant in this region. Moreover, class ephemeral annuals may survive as the dominant type as the stress increasing. The ecological dominance is almost the same as the proportion of species number. This study show if the water-level fluctuation rhythm changed, the composition of annual will change as the response rapidly in the drawdown zone. The original riparian annual plants don't have any advantage in the competitions for survival and spaces in drawdown zone. The adaptive annuals mainly because of that their phenology do not compound with the submergence occurred time then survive as dominant species.