Ag surface plasmon resonance promoted step-scheme (S-scheme) SnNb2O6/Ag3PO4 heterojunctions were constructed via simple chemical deposition and precipitation. The samples were characterized by X-ray diffractometer, field-emission scanning electron microscope, high resolution transmission electron microscope, X-ray photoelectron spectrometer, UV-Vis diffuse reflectance spectrometer and photoluminescence spectrometer. The photocatalytic activities of SnNb2O6, Ag3PO4 and SnNb2O6/Ag3PO4 composites can be determined via degradation experiments. The results show that the SnNb2O6/Ag3PO4 composite possesses an optimum degradation performance in the photocatalytic degradation of methylene blue. Besides, the pseudo-first-order rate constant (Kapp) of SnNb2O6/Ag3PO4 is 0.313 min-1, which is 13.1 times and 4.8 times greater than that of SnNb2O6 and Ag3PO4, respectively. Also, Ag surface plasmon resonance-promoted SnNb2O6/Ag3PO4 S-scheme heterojunction can accelerate the separation of electron-hole pairs, thereby enhancing the redox reaction capability of the whole system.