NOVELTY - Multi-layer composite flat panel type beta -aluminum oxide solid electrolyte comprises a compact layer, and a porous support layer composite. The electrolyte is prepared by e.g. preparing compact layer casting slurry by ball-milling beta -aluminum oxide ceramic powder, solvent and dispersant in a nylon ball-milling tank, adding adhesive and plasticizer, ball-milling and degassing, preparing porous support layer casting slurry by ball milling ceramic powder, pore-forming agent, solvent and dispersant, adding caking agent and plasticizer, secondary ball milling and degassing the slurry.    USE - The multi-layer composite flat panel type beta -aluminum oxide solid electrolyte is useful in sodium-sulfur batteries.    ADVANTAGE - The solid electrolyte: is adhered to porous support layer thin wall, which enhances porous support layer function to prevent solid electrolyte cracking during the usage process and improves the safety performance of sodium-sulfur battery; comprises multi-porous support layer structure, which provides sodium ion transmission channel for transmitting sodium ion and preventing blockage and resistivity to increase the ion flux.    DETAILED DESCRIPTION - Multi-layer composite flat panel type beta -aluminum oxide solid electrolyte comprises a compact layer and a porous support layer composite. The solid electrolyte is prepared by tape casting process by (a) preparing compact layer casting slurry by introducing 15-25 vol.% beta -aluminum oxide ceramic powder, 50-65 vol.% solvent and 1-10 vol.% dispersing agent into a nylon ball-milling tank provided with agate balls, ball-milling once, where the ratio of grinding media to material is (1-3):1, bal milling at a rotating speed of 80-400 revolutions per minute for 8-24 hours, adding 10-21 vol.% adhesive and 5-12 vol.% plasticizer to carry out secondary ball milling to form a stable slurry, ball-milling for 4-12 hours, degassing the stable slurry at a vacuum degree of less than or equal to 0.01 Mpa for 1-4 hours, (b) preparing porous support layer casting slurry by introducing 10-25 vol.% ceramic powder, 3-12 vol.% pore-forming agent, 45-60 vol.% solvent and 1-10 vol.% dispersing agent into a nylon ball-milling tank having agate balls, where the ratio of grinding media to material is (1-3):1, ball-milling at a rotating speed of 80-400 revolutions per minute for 8-24 hours, adding 5-18 vol.% caking agent and 5-12 vol.% plasticizer to carry out secondary ball milling to obtain stable slurry, ball-milling for 4-12 hours, degassing the slurry under vacuum at a pressure of 0.01 Mpa for 1-4 hours, (c) conveying the prepared porous support layer slurry into a tape casting device, base band casting and molding, controlling scraper gap, material groove liquid level height and casting speed to obtain uniform thickness support layer blank band, drying, removing adhesive and sintering, (d) conveying the prepared compact layer slurry into a tape casting device, base band casting and molding, controlling scraper gap, material groove liquid level height and casting speed to obtain uniform thickness multi-layer composite solid electrolyte blank band, drying, removing adhesive and sintering to obtain multi-layer composite solid electrolyte flat panel, and (e) preparing three-layer composite of solid electrolyte, on the other side of the support layer by the step (d).