Pyrite cinder (PyC) was employed as a heterogeneous Fenton-like catalyst, and its catalytic activity was evaluated in view of the effects of catalyst dosage, pH and leaching metal ions. PyC showed significant reactivity, and the pseudo-first-order kinetic rate constant for decomposition of H2O2 and degradation of Acid Red B (ARB) were 3.4 and 14.89 (10−3 min−1) respectively when pH = 5. When 20 g/L PyC was added into 10 mM H2O2 solution in neutral pH, H2O2 could be completely degraded within 4 h, and more than 90% ARB was removed. Leaching metal ions from PyC were found to have little effect on decomposition of H2O2 or on degradation of ARB. PyC still had high catalytic activity after five successive runs. The decomposition mechanism of H2O2 was analyzed and the Haber–Weiss mechanism was employed in this paper. The electron spin resonance image showed •OH was produced and increased between 3 and 5 min in the PyC catalyzing H2O2 reaction, which demonstrated that PyC had a durable ability to produce •OH.