During the life history, the aquatic heteroblastic plant undergoes submerged-leaf and floating/emerged-leaf phases, with dramatic changes in their surrounding environments. However, little is known about the physiological and biochemical changes of the two types of leaves adapting to submerged and aerial environments. In the present study, a heteroblastic plant Ottelia cordata was chosen to investigate the variations in morphology, anatomy, physiology, and biochemistry between the two types of leaves. The results showed that the floating leaf developed stomate to absorb atmospheric CO₂ for photosynthesis to adapt to the aerial habitat, while the submerged leaf showed longer, narrower, and thinner to maximize inorganic carbon uptake underwater. Through external carbonic anhydrase catalysis, both types of leaves could use HCO₃^- as an extra carbon source, and it was stronger in submerged leaves. Besides HCO₃^- use, the δ¹³C and the activity of C4 process key enzymes supported that the submerged leaf could perform C4 metabolism, while the floating leaves only carried on C3 metabolism. This is the first time to report an aquatic plant transiting from the C4 process underwater to the C3 process in the air. Based on the discrimination of stable carbon isotope, in the field, the main carbon source was atmospheric CO₂ for floating leaves and the free CO₂ underwater for submerged leaves. Additionally, HCO₃^- could approximately support 16% carbon source for underwater photosynthesis, which was the second important carbon source for submerged leaves. All our data indicated the heteroblastic aquatic plant adapts to underwater and aerial environments in morphology, anatomy, physiology, and biochemistry for completing life history and maintaining the population in the field.