The effect of peroxyl radical-induced oxidation by thermolysis of 2,2'-azino-bis(2-amidinopropane) (AAPH) on the structural characteristics of rice bran protein fibril (RBPF) and the stability of high internal phase Pickering emulsion (HIPPE) was investigated. The results indicated that the thioflavin T intensity (1306.67 a.u.), average particle size (1208 nm), ζ-potential value (26.31 mV), surface hydrophobicity (1105.81 a.u.), and contact angle (80.7°) of RBPF reached the maximum value when the AAPH concentration was 3 mmol/L, while the α-helix to β-sheet ratio (0.32) of RBPF reached the minimum value. Furthermore, moderate oxidation induced by peroxyl radicals (derived from AAPH concentration ≤ 3 mmol/L) improved the viscoelasticity of RBPF-HIPPE. Correspondingly, RBPF-HIPPE remained stable after 70 days of storage at 25 °C, 1 h of heating at 100 °C, and 4 freeze-thaw cycles. Overall, peroxyl radicals-induced protein oxidation could enhance the stability of RBPF-HIPPE by modulating structural characteristics of RBPF and the rheological properties of RBPF-HIPPE.