The invention discloses a method for preparing a polyimide-titanium dioxide composite nano-film by using a hydrothermal method. The polyimide-titanium dioxide composite nano-film is prepared by the steps of carrying out a hydrothermal reaction, preparing the polyimide-titanium dioxide composite nano-film by using a dip-coating method, separating and washing the polyimide-titanium dioxide composite nano-film, and carrying out heat treatment. The method is simple to operate, good in feasibility and suitable for mass production; furthermore, nano titanium dioxide particles are very good in dispersion uniformity in the film, so that the strength and mechanical properties of polyimide are improved, and the practicality of the polyimide is expanded; therefore, all the properties of the polyimide-titanium dioxide composite nano-film are greatly improved by means of introduction of nano titanium dioxide.