The invention provides a superstable silver-tantalum composite surface-reinforced Raman scattering substrate and a preparation method thereof. According to the invention, a silver-tantalum composite film is deposited through magnetron sputtering; a cleaned single-crystal silicon wafer is vacuumized and deposited with a chromium film; the intensity of current is controlled; the deposition rates ofsilver and tantalum elements are respectively and accurately adjusted and controlled; the reaction pressure of a chamber is maintained; the silver and tantalum elements are deposited through continuous sputtering to form a bimetallic composite material; thus, corrosion resistance and stability of silver are effectively improved; and superstable surface-reinforced Raman scattering detection is realized. The silver-tantalum composite substrate provided by the invention is prepared through magnetron sputtering under a vacuum condition and has pollution-free and high-cleanliness surface, so sensitivity and accuracy of surface-reinforced Raman scattering detection are effectively improved. The superstable silver-tantalum composite surface-reinforced Raman scattering substrate provided by the invention has the advantages of simple preparation process, large specific surface area, low cost, convenient industrial production, and capability of being extensively applied to the fields of surface-reinforced Raman scattering sensing, metal catalysis, nanometer probes, photoelectric devices, solar cells, adsorption materials, etc.