The extensive applications of graphene oxide (GO) inevitably lead to entry into the natural aquatic environment. However, information on its toxicity to emergent plants is still lacking. In this study, an emergent plant,Iris pseudacorus,was exposed to GO (1, 20, 80, and 140 mg·L−1) under hydroponic conditions for 15 weeks. Changes in plant growth were assessed by analyzing plant biomass and photosynthetic pigment contents; the photosynthesis response was verified by measuring chlorophyllafluorescence; and the nutrient levels of the plant were evaluated. Results showed that GO at 20–140 mg·L−1significantly increased plant dry weight by 37–84% and photosynthetic pigment contents by 26–178%, and 80 mg·L−1was the optimal concentration. PSII activity, adjustment capacities of electron transport in PSII, the grouping or energetic connectivity between PSII units, light energy conversion efficiency, photosynthesis performance indexes (by 11–51%), and contents of several nutrient elements (N, Fe, and Cu) were increased by 49–69%, 34–84%, and 11–38%, respectively. These findings indicate that GO can enhance plant growth by promoting plant photosynthesis performance and improving plant nutrient levels, and has great application potential in promoting the growth and development of this emergent plant as a phytoremediation agent.