The invention discloses a subplate for PCB drilling and a preparation method thereof. The subplate is prepared from a wood fiber board, functional water-soluble impregnated paper and thermosetting impregnated paper through a high-temperature high-pressure process. The functional water-soluble impregnated paper is close to the surface of the wood fiber board, so that drilling temperature can be lowered effectively and chip removal of a drilling pin can be promoted in the process of drilling so as to suppress hole blasting; a thermosetting resin layer is positioned on the most superficial layerof the wood fiber board, so that the surface of the subplate is enabled to be high in hardness, and drilling burrs can be inhibited effectively; due to the thermosetting characteristic of the thermosetting resin layer, warping of the subplate due to moisture absorption can be prevented, and storage and transportation are facilitated.