Peroxymonosulfate (PMS) is an alternative to hydrogen peroxide in advanced oxidation processes. Herein, we present a simple one-pot synthetic approach for the Mn-doped graphite phase carbon nitride (g-C3N4) materials and make it as a catalyst to activate PMS for acetaminophen (ACT) degradation. Fabricated Mn-g-C3N4 composites (MnCN) were characterized by SEM, EDS, XRD, XPS, and FTIR. Results showed that Mn was uniformly dispersed in the structure of g-C3N4 mostly in the form of Mn-N coordination. The effects of pH, contents of doped Mn, catalyst dosage, and reusability of catalyst and corresponding kinetic study were conducted to determine the catalytic performance of MnCN. High reaction efficiency was obtained in a wide pH range of 3.2-9. 100% removal of ACT in 15 min under optimized conditions with initial pH 6.5, 0.8 g/L of PMS and 200 mg/L of catalyst of 0.5-MnCN. Additionally, the reaction mechanism of the PMS/MnCN system was investigated using the electron spin resonance technique, quenching studies as well as different organics with electron-donating or electron-withdrawing groups. No PMS decomposition was detected without substrates. No inhibition effect by ethanol or tert-Butyl alcohol and complete inhibition effect by benzoquinone and partly by furfuryl alcohol, as well as ESR spectrum of TEMP-O-1(2) were observed, suggesting that the activation of PMS proceeded in a new way, which is different from traditional sulfate radical-based advanced oxidation processes. A possible mechanism was proposed for ACT removal. PMS was first bounded to Mn-N sites to generate the superoxide anion. Then singlet oxygen produced by superoxide anion would react with ACT by replacing the electron-donating group of acylamino.