The invention relates to a polymethylmethacrylate waterproof paint and a preparation method thereof. The waterproof paint consists of components A and B, wherein the component A is a methacrylic acid resin liquid generated by reaction based on methyl methacrylate (MMA) utilized as a main composition, and the component B is an initiator. When the component A is prepared, a hard monomer, a soft monomer and a cross-linking monomer are put in a reaction kettle and evenly stirred, and then heating is carried out; polymethylmethacrylate moulding plastic is put, and then reaction is carried out for 1-6 hours at the temperature of 70-90 DEG C; cooling is carried out until the temperature is 50-65 DEG C, a polymerization inhibitor, an accelerator, sealing wax and a thixotropic agent are added and evenly stirred; and cooling is carried out until the temperature is 30-45 DEG C, and filtration and packaging are carried out so as to prepare the component A resin liquid. The components A and B are evenly stirred and mixed in a proper proportion, then coated, and finally solidified into a film. The waterproof paint has the characteristics of good strength, good elasticity, low temperature hardness, excellent weather resistance and the like, is simple and convenient for construction, can be solidified rapidly, and can be used for waterproof work of rapid transit railway bridge surfaces and super highway subgrades.