The development of efficient heterogeneous Fenton catalysts is mainly by "trial-and-error" concept and the factor determining H2O2 activation remains elusive. In this work, we demonstrate that suitable facet exposure to elongate O-O bond in H2O2 is the key parameter determining the Fenton catalyst's activity. CuFeO2 nanocubes and nanoplates with different surface facets of {110} and {012} are used to compare the effect of exposed facets on Fenton activity. The results indicate that ofloxacin (OFX) degradation rate by CuFeO2 {012} is four times faster than that of CuFeO2 {110} (0.0408 vs 0.0101 min-1). In CuFeO2 {012}-H2O2 system, OFX is completely removed at a pH range 3.2-10.1. The experimental results and theoretical simulations show that •OH is preferentially formed from the reduction of absorbed H2O2 by electron from CuFeO2 {012} due to suitable elongation of O-O (1.472 Å) bond length in H2O2. By contrast, the O-O bond length is elongated from 1.468 to 3.290 Å by CuFeO2 {110} facet, H2O2 tends to be dissociated into -OH group and passivates {110} facet. Besides, the new formed ≡Fe2+* on CuFeO2 {012} facet can accelerate the redox cycle of Cu and Fe species, leading to excellent long-term stability of CuFeO2 nanoplates.