A new kind of composites of polysilicate aluminium magnesium-caboxy methyl cellulose (PSiAM-CMC) was made with Na2SiO3?9H2O, Al2(SO4)3?18H2O, MgSO4?7H2O and CMC. Jar tests were texted that when the n(Al Mg)/n(Si)=0.6, n(Mg)/n(Al)=1:4, m(CMC)/m(Si)=0.3, gelatinization temperature of CMC 60℃ and pH=2, the PSiAM-CMC flocculant has excellent flocculation performance. The micro-morphology and structure of PSiAM-CMC were studied by scanning electronic microscope (SEM), fourier transform infrared spectrum (FTIR) and X-ray diffraction (XRD). The test results showed that psi, Al, Mg and CMC were composited successfully. The flocculant has got excellent purification effect for Songhua River water.