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"Double-yolk egg-like" Fe 3 O 4 /PBA@PPy nanoreactor is synthesized, where the confinement effect from the cavity, the protective effect from the PPy shell, and the magnetic property were successful integrated together. Therefore, it displayed the high degradation efficiency, low leaching, and rapid separation in Fenton-like reaction. [Display omitted] • k of Fe 3 O 4 /PBA@PPy-10 was 0.38 L·min·mol−1, 5.1 times than that of reference Fe 3 O 4 /PBA-10 in Fenton-like reaction. • The concentration of leached cobalt ions was reduced from 1.47 mg/L to 0.174 mg/L by the protective function from the PPy shell. • 84.5% of the degradation efficiency of the 1st cycle was still maintained after 4 cycles. • The core of Fe 3 O 4 make sure the catalyst can be rapidly separated by a magnet. • Mechanism of Fenton-like reaction is revealed in detail. Peroxymonosulfate (PMS)-based Fenton-like reaction is an effective technique for the pollutant degradation, and the Co-based metal organic frameworks displayed the excellent activity for the PMS activation. Nevertheless, how to further improve the catalytic activity, suppress the leaching of toxic cobalt ions, and realize the rapid separation were still challenges for practical application. In this work, a novel solution was proposed: encapsulating Fe 3 O 4 and Prussian blue analogue (PBA) into the polypyrrole (PPy) shell and constructing a "double-yolk egg-like" Fe 3 O 4 /PBA@PPy as a nanoreactor. In Fe 3 O 4 /PBA@PPy-10, the catalytic performance was remarkably enhanced with the help of confinement effect, and the degradation rate (0.38 L·min·mol−1) was 5.1 times than that of reference Fe 3 O 4 /PBA-10 (0.074 L·min·mol−1). In addition, the concentration of leached cobalt ions was reduced to only 0.174 mg/L by the protective function from the PPy shell. Moreover, the nanoreactor could be magnetically separated from the reaction solution due to the encapsulation of Fe 3 O 4 nanospheres, and 84.5% of activity still preserved after the 4th cycle. The main active species involved in Fe 3 O 4 /PBA@PPy-10 system was 1O 2 , while that in reference Fe 3 O 4 /PBA-10 system was OH. Electron spin resonance analysis and radical trapping experiment revealed that the different catalytic mechanisms were attributed to the confinement effect inside the hollow cavity. This work not only presents a feasible way to prepare rarely-reported double-yolk egg-like nanoreactor, but also provides a new insight to solve the bottlenecks in Fenton-like reaction. [ABSTRACT FROM AUTHOR]