A sound insulation floor slab for building and a construction method thereof pertain to the technical fields of building material components and construction. The sound insulation floor slab is characterized by comprising an upper-layer ordinary cement mortar leveling layer and a lower-layer damping mortar floated coat, the damping mortar floated coat is made by laying acoustic damping mortar which comprises the following components by weight percentage: 60%-80% of sand for construction, 5%-20% of rubber grains, and 10-20% of fine powder for construction. The laying method of the sound insulation floor slab is as follows: firstly impurities and sharp objects on the substrate are cleaned off to ensure that the substrate is flat, secondly the damping mortar floated coat is laid and protected by a cement protection layer, and finally the ordinary cement mortar leveling layer is laid on the cement protection layer. The sound insulation floor slab has the advantages of extremely low raw material cost, simple construction process, easy control on the thickness of sound insulation damping layer, and improving the sound insulation for impact sound by about 12dB, and the sound insulation floor slab meets the sound insulation requirement of high performance buildings.