Rational construction of the facet engineering over metal–organic frameworks is of significant interest for enhancing photocatalytic performance, yet the role of modulator except regulating facet is largely ignored. Herein, facet engineering of NH₂-MIL125 (aMIL) was achieved through the facile one-pot method by controlling the concentration of acetic acid modulator. The probable domino effects induced with the detectable modulator were extensively investigated, evidencing the multi-position in one mode contained powder X-Ray diffraction (PXRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), etc. Meanwhile, correlation among the {111} facets engineering, the degree of structural defects, and the performance of photocatalytic hydrogen peroxide (H₂O₂) production was studied in detail, revealing that facet and defect engineering respectively play positive and relatively negative roles in the photocatalytic oxygen reduction reaction (ORR) with a volcano-type trend. aMIL-3 photocatalyst could deliver H₂O₂ production rate of 925.8 μmol·h⁻¹·g⁻¹ (2.03-fold of aMIL) under visible-light irradiation and a quantum yield of 1.08% at 420 nm.