This paper aimed to investigate the effects of the different existent forms of alginic acid (AA), a polysaccharide organic matter, on the silicate scalings of polyamide composite nanofiltration membranes. Different existent forms of AA in nanofiltration process coexist with saturated silicate in the feedwater, and hence generate organic fouling layers on the membrane surface. The trend of membrane flux decline during silicate scaling was studied by filtration experiments and the structures of the fouling layers on membrane surface were analyzed by scanning electron microscopy and atomic force microscopy. The effect of electrostatic repulsion was investigated through zeta potential. Atomic force microscope combined with silica colloidal probe were used to analyze the characteristics of the adhesion forces during different scaling stages. The results showed that the degree of silicate scaling decreased in the order of virgin membrane > coexistence of AA and silicate (virgin membrane) > AA-fouled membrane. Compared with the silicate scaling of the virgin membrane, AA could weaken the silicate scaling when AA coexisted with silicate. However, when the AA fouling layer had formed on the membrane surface, the degree of silicate scaling was significantly reduced. The above results indicated that the effects on silicate scalings brought by different existent forms of AA vary greatly.