The invention relates to a red sandstone surface modified polyurethane material smeared in a water level variation zone and a preparation method thereof. The material comprises a component A and a component B, wherein the component A comprises a sodium silicate solution, a catalyst, a plasticizer and nano titanium dioxide; and the component B comprises diisocyanate (MDI), polypropylene glycol (PPG) and a waterproof agent. The material has the advantages of good reinforcing performance, no toxicity and the like of a traditional polyurethane reinforcing material, the water resistance of the material can be effectively improved by adding the waterproof agent, and the material is tightly attached to the surface of red sandstone for water isolation, so that the water environment is isolated, and degradation of the physical and mechanical properties of the red sandstone is inhibited; and finally, considering that the problems that red sandstone is subjected to a large amount of solar ultraviolet radiation for a long time in a dry season in a water level variation zone, water loss of the red sandstone is caused, the surface of the red sandstone falls off, the interior of the red sandstonecracks, and finally, the physical and mechanical properties of the red sandstone are deteriorated, the ageing resistance of the red sandstone can be effectively improved by adding the nano titanium dioxide.