This paper compares the difference in membrane flux between direct membrane filtration and coagulation-membrane filtration, and investigates the removal of organics of different molecular weight by coagulation-membrane filtration process, in turn studies the fouling removal mechanism of coagulation process. The results indicate that for COD removal from raw water, coagulation is effective mainly on the organics of molecular weight higher than 10 kDa, and membrane filtration is effective mainly on the organics of molecular weight between 30 kDa and 100 kDa and higher than 100 kDa; as for UV₂₅₄ removal, coagulation is effective mainly on the organics of high molecular weight, and membrane filtration is effective mainly on the organics of molecular weight between 1 kDa and 100 kDa. Therefore, it proves that the high molecular organics are mainly responsible for the membrane fouling.