Persulfate-based advanced oxidation process is a powerful countermeasure for water remediation, where effective and low-cost catalysts are still needed. Herein, a one-pot synthetic method for magnetic nitrogen-doped sludge biochar (MS-biochar) was presented, which exhibited better catalytic property with PDS for tetracycline degradation than typical graphitic carbon (graphite powder, graphene oxide and multiwalled carbon nanotubes) and two other types biochars. EDAX manifested the uniform dispersion of elements in MS-biochar. Similar to "top-down", a research thought from whole to part of MS-biochar for contribution measurement was presented, where acid-soluble substance (ASS) was the most important contributor for MS-400/600 while carbon matrix (CM) was dominant for MS-800. Quenching and EPR demonstrated a free-radicals pathway in MS-biochar/PDS, where ASS mainly assumed to be effective for SO4 center dot- and CM was primarily committed to center dot OH generation. EDAX, XPS and Raman studies proved three kinds of catalytic sites, namely the iron compounds, doped nitrogen and graphitic carbon. And their activating mechanism were discussed where one internal electron migration path (from sp(3) to nanocrystalline sp(2) carbon) has been first proposed. Reusability, metal leaching detection and pharmaceutical wastewater application indicated the potential of MS-biochar. This work not only presents a potential resource-based disposal of sewage sludge, a novel research thought from whole to part for materials performance measurement, but also provides guidance for carbon materials' design for persulfate activation, especially for sp(2) and sp(3) co-hybridized carbons.