Pollution with rare earths has become an important factor restricting agricultural development. The objective of this study was to analyze the photosynthetic physiological mechanism of exogenous hydrogen peroxide (H₂O₂) mitigating lanthanum (La) induced stress damage in naked oat (Avena nuda) seedlings. Seedlings of naked oat cultivar ‘Baiyan 7’ were used in a sand culture experiment to investigate the effect of spraying 5 mmol/L H₂O₂ on seedling growth, leaf chlorophyll fluorescence parameters, and photosynthetic carbon assimilation enzyme activities upon a 1.20 mmol/L La³⁺ treatment. The results showed that root length, plant height, and biomass, and the leaf de-epoxidation extent of xanthophyll cycle (A+Z)/(V+A+Z) of naked oat seedlings sprayed with the H₂O₂ decreased significantly, and PSⅡ maximal photochemistry efficiency (F_v/F_m),practical photochemical efficiency(Φ_PSⅡ),photochemical quenching coefficient (qP), and quotient of absorbed luminous energy used for photochemical action (P) apparently increased. However, PSⅡ non-photochemical quenching coefficient (NPQ), regulated energy dissipation Y(NPQ), non-regulated energy dissipation Y(NO), quotient of absorbed luminous energy used for antenna heat dissipation (D), quotient of absorbed luminous energy used for non-photochemical dissipation (E_x) in reaction center of PSⅡ, and unbalanced distribution of coefficient deviation on excitation energy between two photosystems (β/α-1) decreased significantly under La stress. Moreover, the activities of ribulose 1, 5-biphosphate carboxylase (Rubisco), sedoheptulose-1,7-bisphosphatase (SBPase), and fructose-1,6-bisphosphate aldolase (FBAase) increased significantly. However, there was no significant change in transketolase (TKase) activity. These results show that exogenous H₂O₂ can decrease photoinhibition caused by La stress through improving PSⅡ photochemical efficiency and carbon assimilation key enzyme activity, but not based on the xanthophyll cycle-dependent energy dissipation, which showed to mitigate inhibition of La stress on seedling growth and enhanced adaptability of naked oat to La stress.