Display omitted •MMT-nano CaO2 was first developed to enhance dewatering of waste-activated sludge.•Montmorillonite has the catalytic capacity for generating OH from nano CaO2.•MMT-nano CaO2 exhibited synergistic effects of filtration aid and peroxidation. Nanosized CaO2 was first synthesized using montmorillonite clay layers as templates for combining the positive effects of filtration aid and peroxidation on the dewatering of waste-activated sludge (WAS). Exchangeable Ca2+ compensating for negative charges of montmorillonite reacted with the added H2O2, affording nano CaO2 on the surface or in the interlayer regions of montmorillonite (montmorillonite-supported nano CaO2, MMT-nano CaO2). The catalytic capacity of montmorillonite for generating OH from nano CaO2 was verified by electron paramagnetic resonance (EPR), therefore, the radical oxidation induced by MMT-nano CaO2 can destruct the floc structure of WAS with the omission of Fe(II), the conventionally used catalyst for Fenton reaction. As a result, the reduction in specific resistance of filtration (SRF) by 62.2% was achieved with the MMT-nano CaO2 dosage of 100mg/g dry solid (DS) and the weakly acidic pH of 5. OH led to the reduction of hydrophilic protein in extracellular polymeric substances (EPS), and the swelling behavior of montmorillonite contributed to the formation of a porous structure as draining medium for water withdrawing. The above factors were the dominant mechanisms for enhancing dewatering performance.