The invention discloses a low-compression permanently deformed PPDI (p-phenylene diisocyanate)-based polyurethane elastomer and a preparation method therefor. The preparation method comprises: carrying out a reaction on organic polyalcohol and PPDI to obtain an isocyanate group-terminated prepolymer with the content of NCO being 2-9%; and then carrying out a reaction on the prepolymer and a mixture of a small molecule diol chain extender with the mass fraction being 10-60% and a small molecule triol chain extender with the mass fraction being 40-90% to produce a casting type PPDI-based polyurethane elastomer which is relatively good in mechanical property (the tensile strength is greater than 10MPa) and relatively low in compressed permanent deformation (the compressed permanent deformation is less than 8%). The elastomer is particularly suitable for the specific medical field which is subjected to the compressing force and requires a shock absorption function.