The invention discloses plastic concrete for a flow channel cushion layer and a preparation method for the plastic concrete, and relates to the field of preparation of structural concrete for water conservancy projects. The plastic concrete comprises the following raw materials in parts by weight: 286 parts of a cementing material, 1695 parts of an aggregate, 0.419 part of a water reducing agent and 254 parts of water, wherein the sand rate of the aggregate is 45%; and the cementing material comprises the following raw materials: 86 to 114 parts of cement, 72 to 100 parts of fly ash and 86 to114 parts of bentonite. The plastic concrete provided by the invention has proper strength, elasticity modulus and impermeability, and has excellent adaptability to the construction environment of theflow channel cushion layer.