The invention discloses a polyurethane soft foam material for curing ballast beds and a preparation method thereof. The polyurethane soft foam material is prepared from following raw materials: a material A, a material B and a catalyst material C, wherein the material A includes the following raw materials: polyether polyol, a small-molecular chain extender and a foaming agent; the material B is a pre-polymer prepared from isocyanate and polyether polyol; the material C is a catalyst and the weight ratio of the material C to the material A is 1.2-1.4:100. The catalyst is stannous ricinelaidate. In a moulded sample being not more than 350 kg/m<3> in density, the polyurethane soft foam is not less than 0.6 MPa in tensile strength, is not less than 200% in break elongation rate, is not less than 500 N/m in tear strength and is not less than 25 kPa in compression strength. The polyurethane soft foam material has excellent deformation recovery situation with compression set being not more than 10% and satisfies an aging standard, has more convenient construction advantages on the basis of activity and foam rising height of materials being casted in railway ballasts at different temperatures.