The invention provides a flexible composite substrate for a perovskite thin-film solar cell and a preparation method thereof. The flexible composite substrate has the characteristics and functions ofultra-thinness, weather resistance, water and oxygen barrier, transparent conductive films and the like, and solves the problem that existing PET, PEN and other base materials are poor in weather resistance. Besides, the water-oxygen blocking layer can prevent water vapor and oxygen in the environment from corroding the perovskite cell, and the requirement for long-term use of the cell in the outdoor humid and hot environment is met. The flexible composite substrate for a perovskite thin-film solar cell and a preparation method thereof can break through the preparation technology of the ultrathin high-barrier layer, integrate the barrier layer, the ultrathin base material and the transparent electrode into one body, reduce the use of a layer of packaging film, reduce the surface density ofthe battery, simplify the traditional sandwich type battery packaging process, only need a one-side pressing process, can reduce the side leakage risk of the device, and can improve the long-term stability of the device.