The present invention provides a porous thermal insulation material capable of designing and enhancing the thickness and the density, and a preparation method thereof. The porous thermal insulation material comprises quartz fibers, mullite fibers, starch, boron nitride and phenolic resin powder. The method comprises: (1) preparing and processing a porous thermal insulation material as a matrix; (2) carrying out pre-packaging treatment on the periphery of the matrix material; (3) determining the quality of a phenolic resin impregnating solution; (4) adding the porous thermal insulation material obtained in the step (2) to the phenolic resin impregnating solution obtained in the step (3), carrying out normal pressure impregnating, and then carrying out air drying; and (5) carrying out pressurization curing on the material obtained in the step (4) to obtain the porous thermal insulation material. According to the present invention, the enhanced thickness of the material can be adjusted, the internal enhanced thickness and the external enhanced thickness are uniform, and the stable and reliable effect of the preparation process is achieved; the problem that different shrinkages of the impregnated region and the unimpregnated region after heating easily cause the material deformation and crack is avoided, and the reinforced thermal insulation material with characteristics of flatness and no layering is obtained.