Verifying the role of H2O2 is a critical challenge for deepening the understanding of the mechanism of peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs). In this research, Co, Fe bimetallic organometallic frameworks (ZIF-L@Fe28) were prepared, and their ultrahigh activities via H2O2 generation at Fe and Co sites were experimentally and theoretically verified. The important contribution of H2O2 to 1O2 production was identified through the quantifications of the mutual transformation of active species. Co sites were identified as the most active positions for PMS activation because of they strongly triggered 1O2, SO4·−, ⋅O2−, ·OH, and H2O2. However, the self-quenching of active species suppressed the efficiency and selectivity of 1O2 generation at Co sites. In contrast to Co sites, Fe sites had selective preference for H2O2 generation, and the transformation of H2O2 to 1O2 further enhanced the performance of ZIF-L@Fe28. The supplementary discussion from the perspective of H2O2 will enable the further application of the PMS activation mechanism.