In this paper, the pre-magnetization of sludge was carried out using electromagnetic field, and the effect of electromagnetic field intensity and time on the coagulation efficiency was investigated. The results indicated that the turbidity and DOC removal efficiency of recycling magnetized sludge can be improved, the removal of which was 3.7% and 32.4% respectively higher than the cases without electromagnetic field. Sufficiently large magnetic field intensity and magnetization time played a significant effect on the enhancement of the coagulation efficiency. The Zeta potential of the mixed water and sludge increased by electromagnetic field and the flocs characteristics were obviously improved, accordingly the flocs had bigger growth rate, average flocculation index and average flocs size with denser and regular structure.