The phthalocyanine precursor 3-octylaminophthalonitrile (3-nOPn) and the tin dioxide precursor Sn(OH)4 were synthesized. The two precursors were synthesized into a new type of composite material by in-situ synthesis, 3-nOCoPc/SnO2. Through X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and N2 physisorption measurements the material was characterized. Using the synthesized composite material for photocatalytic degradation of Rhodamine B dye, degradation reached 92.38%, better than using 3-nOCoPc or SnO2 alone. Experiments proved that the new composite material has good degradation ability, good recyclability, and potential application value.