Vallisneria spirals (V. spiralis) was used to reveal how the light intensity at the top of the plant, plant morphology, and antioxidant enzyme activity respond to different hydrologic conditions from Lake Poyang, China. By designing a laboratory experiment simulating historical water levels of low, normal, and high wetland plant submersion, this study aimed to elucidate the effects of different levels of flooding on growth and antioxidant enzyme activity for V. spiralis. The results showed that the plant crown light intensity of the treated group and control group first decreased and then increased along with the seasonal variation of the water level. The maximum and minimum values of the plant crown light intensity were observed in April and July, respectively. Similar to the control group, V. spiralis from the normal and low water level groups was measured and had higher plant height growth in the flooding period from May to June, and the entire plant biomass also showed a steady growth trend in the same period. However, the plant growth of the high water level group was lower during the whole simulation period, with negative growth in July. Antioxidant enzyme activities changed with the seasonal temperature, and the activity of the control group showed a rising trend. Compared with those of the control group, the antioxidant enzyme activities of the high water level group showed a “bell shaped” trend, which was first significantly induced and then significantly inhibited. In addition, the peroxidase (POD) and catalase (CAT) activities from the low water level group in April were also significantly induced. The IBR index showed that a comprehensive biological index could well reflect the effects of seasonal water levels in Poyang Lake on the growth of the wetland plant V. spiralis. This study indicated that high flooding levels had the strongest negative effect on the growth and enzyme activity of the submerged plant V. spirals.