The invention discloses a self-compacting concrete for rapid transit railway III-type plate ballastless tracks and a preparation method of the self-compacting concrete. The self-compacting concrete can effectively eliminate bleeding in a pouring process, has low contractibility rate and good durability, and is beneficial to improving of pavement quality of track plates and prolonging of service life of the ballastless tracks. The self-compacting concrete is prepared by stirring and mixing cement, sand, stone, an expanding agent, a water reducing agent, a shrinkage reducing agent, a thickening agent, a water reducing agent, a collapse preventing agent and water as raw materials, wherein the raw materials comprise the following components in parts by weight: 100 parts of common silicate 425# cement, 11 parts of an expanding agent, 38.5 parts of coal ash, 20.5 parts of mineral powder, 261-277 parts of sand with fineness modulus of 2.4-2.8, 235-251 parts of stone with a grain size of 5-10mm, 0.04 part of a thickening agent, 56-60 parts of water, 1.3 parts of a shrinkage reducing agent, 2.3 parts of a water reducing agent and 0.37 part of a collapse preventing agent.