The invention provides a barium strontium titanate nanoparticle modified titanium dioxide thin film hydrogen sensor and a preparation method. The preparation method includes: combining a magnetron sputtering method, a spin-coating method and a hydrothermal method, and regulating spin-coating size and dispersity of barium strontium titanate nanoparticles to realize modifying of the surface of a titanium dioxide thin film; using a magnetron sputtering method to prepare a titanium dioxide seed crystal layer on an FTO substrate, using the hydrothermal method to prepare the titanium dioxide thin film on the seed crystal layer, adopting the spin-coating method to spin-coat the surface of the titanium dioxide thin film with the barium strontium titanate nanoparticles prepared by the hydrothermal method in advance, annealing after each step of preparation, and finally using the magnetron sputtering method to prepare a metal platinum electrode on the surface the titanium dioxide thin film. Sensitivity in hydrogen detection of the titanium dioxide thin film modified by the barium strontium titanate nanoparticles at room temperature is remarkably improved. The hydrogen sensor is low in preparation cost, the preparation method is simple and easy to operate, and the hydrogen sensor prepared by the method is high in hydrogen detection sensitivity at room temperature and high in repeatability and stability and can be widely applied in the field of hydrogen new energy resources.