To put forward a Fenton-like system based on a new type of Fe₃O₄ magnetic nanoparticles catalyst, Fe₃O₄ magnetic nanoparticles were prepared using chemical co-precipitation , and their surface was modified with tetramethyl ammonium hydroxide (TMAH). The catalyzed degradation of phenol by Fe₃O₄-H₂O₂ Fenton-like system was investigated, including the effect of catalyst dosage, H₂O₂ dosage, pH value and reaction time on the removal of COD and volatile phenol. The results showed that the mean size of the Fe₃O₄ nanoparticles is 30 nm and they show a good disparity between 20 and 100 nm. Three different doses of TMAH coated catalysts after ultrasonic pretreatment, revealed almost the same treatment efficiency of 50 mg/L phenol (112 mg/L COD equivalent) under the room temperature (13 ℃). Under the condition of 0.8 mmol/L catalyst dosage, 2.0 mmol/L H₂O₂ dosage, pH 4.5 and the reaction time of 180 minutes, the highest COD removal efficiency was 72%; while under the condition of 0.4 mmol/L catalyst dosage, 2.0 mmol/L H₂O₂ dosage, pH 4.5 and the reaction time of 90 minutes, the removal rate of volatile phenol was close to 100%. 3# Fe₃O₄-TMAH（2 mL）catalysts revealed optimal reuse efficiency, the removal rate of COD were 73%,29%,28%,26%, the removal rate of volatile phenol were 100%,84%,67%,54%. The Fenton-like system has the advantage of no redundant excess sludge producing, and the magnetic catalyst can realize quick separation and recycling under the action of outside magnetic field.