The invention discloses anionic emulsified modified asphalt and a preparation method thereof and anionic high-adhesiveness waterborne asphalt waterproof coating. The anionic emulsified modified asphalt is prepared from, by mass, 50-70 parts of matrix asphalt, 1-20 parts of a polymer modifier, 0.5-2 parts of nano carbon fibers, 2-5 parts of a surface modifier, 1-2 parts of an anionic surfactant, 0.1-0.8 part of a pH regulator and 20-40 parts of water. A film formed by the waterproof coating has excellent creep performance and extending performance and is capable of well sealing cracks of a baselayer to effectively resist cracking caused by base layer settlement, deformation and the like, water channeling is prevented, and reliability of an upper waterproof roll is improved.