A ultrafine carbon-supported Mn3O4 nanocatalyst by poly (vinyl alcohol)(PVA) mediated precipitation method using poly(ethylene glycol) (PVA) as a stabiliser was reported. The structure and morphology of the prepared nanomaterials were characterized by XRD, XPS, and TEM, and it was found that PVA could effectively reduce the aggregation and growth of Mn3O4 nanoparticles during the preparation process. The size of Mn3O4 nanoparticles prepared in the absence of PVA is 38.8 (±9.3) nm, while the size of Mn3O4 nanoparticles dispersed in the PVA molecular chains was 3.2 (±0.8) nm. After the Ar-protected carbonization treatment of the freeze-dried PVA/Mn3O4 composite, the Mn3O4 nanoparticles were uniformly immobilized on the C substrate with a particle size of 4.5 (±1.2) nm. The prepared Mn3O4-C catalysts have excellent ability for Fenton-like degradation of methylene blue (MB). Meanwhile, Mn3O4-C has better cycling stability compared to the Mn3O4 nanoparticles prepared in the absence of PVA.