<p><b>OBJECTIVE</b>To explore changes of B and T lymphocyte attenuator (BTLA), superoxide dismutase (SOD), catalase (CAT), total antioxidant capacity (TAOC), reactive oxygen species (ROS), reactive nitrogen species (RNS), malondialdehyde (MDA) in ankylosing spondylitis (AS) patients, and the effect of Xinfeng Capsule (XFC) on them.</p><p><b>METHODS</b>Totally 120 AS patients were assigned to two groups according to random digit table method, the XFC group (3 XFC pills each time, 3 times a day) and the SASP group (4 SASP tablets each time, twice a day), 60 in each group. All patients were treated for 3 months. Another 60 healthy subjects were recruited as a healthy control group. The expression frequency and activation levels of BTLA were detected using flow cytometry. Serum oxidative stress indices (such as SOD and CAT, TAOC, ROS, RNS, MDA) and contents of cytokines [tumor necrosis factor α (TNF-α), IL-1β, IL-4, and IL-10] were detected using enzyme-linked immunoassay (ELISA). Erythrocyte sedimentation rate (ESR) was detected using Westergren method. High-sensitivity C-reactive protein (Hs-CRP) was detected using HITACHI 7060 type automatic biochemical analyzer. Clinical efficacies of ASAS 20 and BASDAI50 were assessed using VAS. Correlation analysis between scoring for quality of life and BTLA expression frequency was performed.</p><p><b>RESULTS</b>(1) Clinical efficacies of ASAS 20 and BASDAI50 were significantly better in the XFC group than in the SASP group (P < 0.01). (2) Compared with the healthy control group, BTLA expressions in the peripheral blood of AS patients decreased significantly (P <0. 05); SOD, CAT, and TAOC values significantly decreased (P < 0.01, P < 0.05); ROS, RNS, and MDA values significantly increased (P < 0.01, P < 0.05); TNF-α, IL-1β, ESR, and Hs-CRP values significantly increased (P < 0.01); IL-4 and IL-10 values decreased significantly (P < 0.01, P < 0.05). (3) Compared with pre-treatment in the same group, BTLA/CD19 + B, BTLA/CD24 + B, SOD, TAOC, IL-4, SF-36 [physical functioning (PF), social functioning (SF), role limitation due to physical problems (RP), role limitation due to emotional problems (RE), body pain (BP), mental health (MH), vitality (VT), general health (GH)] were significantly elevated; ROS, MDA, TNF-α, ESR, Hs- CRP, VAS, BASDAI and BASFI, BAS-G were significantly lower in the peripheral blood of the two groups after treatment (P < 0.01, P < 0.05). Better effect was shown in the XFC group in elevating BTLA/CD19+ B, BTLA/CD24 + B, SOD, TAOC, IL-10, BP, MH, VT, and SF; and lowering ROS, IL-1β, MDA, TNF-α, ESR, Hs-CRP, VAS, BASDAI, BASFI, and BAS-G (P < 0.01, P < 0.05). (4) Pearson correlation analysis showed, BTLA/CD19 + B expression of the peripheral blood was positively correlated with SOD, CAT, TAOC, IL-4, IL-10, GH, RP, BP, and SF (r = 0.431, 0.325, 0.318, 0.316, 0.348, 0.314, 0.358, 0.318, 0.326, respectively, P < 0.05, P < 0.01), while it was negative correlated with ROS, MDA, TNF-α, IL-1β, ESR, VAS, and BASDAI (r = -0.342, -0.368, -0.334, -0.354, -0.324, -0.372, -0.342, respectively, P < 0.05, P < 0.01). BTLA/CD24 B expression of the peripheral blood was positively correlated with SOD, TAOC, IL-4, IL-10, GH, RP, BP, SF, RE, MH, VT (r = 0.358, 0.352, 0.372, 0.436, 0.435, 0.326, 0.352, 0.345, 0.326, 0.343, 0.332, respectively, P < 0.05, P < 0.01), while it was negative correlated with ROS, RNS, MDA, ESR, Hs-CRP, VAS, BASDAI, and BASFI (r = -0.447, -0.336, -0.405, -0. 395, -0. 358, -0.436, -0.338, -0.425, respectively, P < 0.05, P < 0.01).</p><p><b>CONCLUSION</b>XFC could improve BTLA expression in the peripheral blood of AS patients, negatively regulate activation and proliferation of B cells, and reduce abnormal immune responses and oxidative stress injury, thereby effectively alleviating joint stiffness and pain.</p>