The invention provides a preparation method of an epitaxial-growth potassium-sodium niobate-barium zirconate-sodium bismuth titanate lead-free piezoelectric thin film having a vertical phase boundary. A chemical solution precipitation process is adopted, a KNN-BZ-BNT precursor solution is doped with manganese acetate, cobaltous acetate and ferrous chloride to reduce leak current density in the thin film and to improve the electrical performance of a KNN-BZ-BNT film, then the precursor solution is precipitated on a monocrystalline SrTiO3 substrate in three different orientations (100), (110) and (111), a monocrystalline LaAlO3 substrate in orientation (100) and a monocrystalline SrRuO3 substrate in orientation (100) by adopting a spin-coating process, and the substrates are thermally treated to obtain a compact crack-free single-layer monocrystalline thin film having an epitaxial characteristic. After the spin coating-thermal treatment process is repeated for multiple times, the epitaxial-growth KNN-BZ-BNT lead-free piezoelectric thin film with the thickness of 100nm to 600nm and having the vertical phase boundary is obtained.