Abstract: To understand the impacts of rhizosphere oxygen environment in wetland plants in response to flooding stress, a typical wetland plant, Acorus calamus, was selected to study its rhizosphere oxygen characteristics in response to flooding stress. Young seedlings of height around 15 cm and adults plants of height 40 cm were cultivated in sediments and flooded completely. Rhizosphere oxygen profiles at the middle part of the whole root and inside the root axis were measured using a micromotor-controlled oxygen micro optode for 3 weeks. The results showed that at day 7 of the flooding period, both young seedlings and adult plants demonstrated a great reduction of thickness in the rhizosphere oxygen diffusive layer, while at day 14 of the flooding period, rhizosphere oxygen diffusion in young seedlings totally disappeared while that in the adult plants was partially maintained, and at day 21 of the flooding period, the thickness of oxygen diffusive layer in adult plants was 0.34 mm and the oxygen saturation rate was 11.18%. The thickness of oxygen diffusive layer and the average oxygen saturation rate in flood-treated young seedlings and adult plants significantly declined, in comparison to the declining root endogenous oxygen concentration with an even greater decrease rate. At the end of the experimental period, the three parameters mentioned above in flooded young seedlings decreased from 0.22 mm, 1.4% and 50.3% to 0 mm, 0% and 32.9%, while those in adult plants decreased from 0.64 mm, 19.3% and 64.6% to 0.34 mm, 11.2% and 55.3%, respectively. The root endogenous oxygen concentrations in both young and adult flooded seedlings were mainly affected by the photosynthetic rate, however, those in the control plants were affected by the photosynthetic capacity and gas exchange comprehensively. A short-term flooding stress of seven days could stimulate the development of aerenchyma in the root of A. calamus, which increased the root porosity from 28.45% to 32.44% and was beneficial to maintaining the root endogenous oxygen concentration, which had no significant difference between flooding group (65.6%) and control group (67.1%). The root endogenous oxygen concentrations of A. calamus were the key factors influencing radial oxygen loss under flooding stress. There was no oxygen diffusion effect in flooded young seedlings, when the root endogenous oxygen concentrations was so low that it threatened plant survival, while those in adult plants remained a certain level due to a higher root endogenous oxygen concentration.