The invention relates to a fiber composite material with a microwave absorbing function and a preparation method thereof. The invention is characterized in that: continuous silica fibers (SiO2f) are introduced to serve as a mechanical property reinforcement and electromagnetic wave absorbing medium, and a matrix material is a SiO2 material or a wave absorbing agent SiO2 material. The content of the continuous SiO2f is 10 to 40 weight percent and the diameter of the continuous SiO2f is 4-15mu m. In the preparation process, polytetrafluoroethylene or organic silicon resin is used as a fiber coupling agent, and polyvinyl acetate (PVA) or polyvinyl butyral (PVB) is used as an adhesive. A composite blank is prepared by the traditional impregnation twisting method, and a hot pressed sintering method is adopted for forming and sintering. The prepared SiO2f/SiO2 composite material has the characteristic of structure and function integration; and the prepared fiber composite material has the microwave shielding attenuation loss reduced from -4dB to -35dB in a frequency band of 8-12GHz compared with a pure molten SiO2 block, keeps invariable return loss and reflection but better shielding effect and is suitable to be used as a wave absorbing material matrix.