The invention relates to a preparation method of a gradient density resin composite preform and belongs to the technical field of ablation thermal protective materials of hypersonic flight vehicles. Aprepared non-vitrified resin solution and vitrified resin dissolving pulp are paved on quartz fiber woven cloth respectively, a-type micro-vitrified prepreg, b-type micro-vitrified prepreg, c-type micro-vitrified prepreg and d-type micro-vitrified prepreg are prepared respectively after drying, the prepregs are paved in a forming cavity of a paving and stitching tool, and a finished product of the gradient density resin composite is prepared with a vacuum-pumping compacting method. The prepared gradient density material is divided into multiple layer in the aspect of plane composition, all layers of materials are in uniform transition, so that the interface problem is eliminated, the requirements of a thermal protection system of a hypersonic flight vehicle for an anti-ablation and anti-shear surface layer, a low-density and efficient thermal-insulating inner layer and a buffering and transition middle layer are realized, and the requirements of lightweight, low ablation and efficientthermal insulation of the thermal protection system of the hypersonic flight vehicle are met.