The invention discloses cement asphalt mortar and a preparation method thereof. The cement asphalt mortar comprises emulsified asphalt, cement, fine sand, an expanding agent, aluminium powder, and water. The formula of the emulsified asphalt comprises asphalt, an anionic emulsifier, a cationic emulsifier, a stabilizing agent, a thickening agent, a water reducer, an antifoaming agent, alcohol, polymer latex and water. The cement asphalt mortar of the invention adopts specially prepared emulsified asphalt; when the emulsified asphalt is mixed with dry materials of cement, dry sand, and the like, the water reducer, the antifoaming agent, and the polymer latex are easy to disperse uniformly, and the contact with the dry materials is full; the water reduction effect is significant; the bleeding phenomenon of the CA mortar after pouring is effectively prevented; the workability of the emulsified asphalt with cement and fine sand is improved; with the synergistic effect of the alcohol, the freezing resistance of the CA mortar is improved; big bubbles are eliminated effectively; the CA mortar is compact and uniform in texture after molding, and has the characteristic of good weatherability. The cement asphalt mortar of the invention is especially suitable for being poured below a track plate during high speed railway building and used as a damping buffer layer.