Impact of elevated O3 (80 nmol mol−1) was studied on antioxidant system in the previous-year needles (P-needles) of Pinus armandii under the background of global climate change. CAT (catalase) did not play an important role in defensing O3 pollution, as no significant change was observed in its activity in P-needles exposed to elevated O3 during the entire growth season. Ascorbate (AsA) was more sensitive to elevated O3, as the O3-induced decrease in its content was observed at Day 45. It suggests that AsA consumption was a crucial way for protecting against O3 pollution. After 75 days of O3 exposure, increase in superoxide dismutase and ascorbate peroxidase activities occurred in P-needles (p < 0.05), whereas glutathione reductase activity was significantly increased after 105 days of O3 treatment (p < 0.05). Dehydroascorbate reductase and monoascorbate reductase activities also increased after 75 days of elevated O3 exposure. However, the elevated antioxidant capability did not suppress the generation of reactive oxygen species, and as a result, increase in malondialdehyde content was noted (p < 0.05). Noticeably, no change was observed in electrolyte leakage during the entire growth season under elevated O3 fumigation. It indicates that O3-induced oxidative stress was not severe in the needles of P. armandii. In other words, P. armandii has certain tolerance to O3 pollution.