An in-situ cast floor underlayment is disclosed in this invention, wherein the floor underlayment comprises a cemented rubber layer, which further comprises rubber granules and cementitious binder. The floor underlayment further comprises a highly deformable cemetitious composite layer on top of the cemented rubber layer for the protection thereof. The rubber granules are pre-treated such that a better interaction between the rubber granules and the cementitious binder is achieved. A method of installing the in-situ casted floor underlayment is also disclosed.