The invention provides porous silicon-based structural ceramic and a preparation method thereof. The method comprises steps as follows: nano SiO2 powder and C powder in the mass ratio being (50-100):(0-50) are subjected to ball milling and mixed, and basic powder is obtained; the basic powder and an organic solvent are subjected to ball milling and mixed, and prepared slurry with the solid phase content being 50wt%-80wt% is obtained; the prepared slurry and a photoinitiator are mixed and formed with a photo-curing additive manufacturing technology, a ceramic biscuit is obtained, after adhesive removal is performed in a vacuum atmosphere, the ceramic biscuit is subjected to reaction and sintered in an argon and/or nitrogen atmosphere, and the porous silicon-based structural ceramic is obtained. The method is based on the photo-cured additive manufacturing technology, the porous silicon-based structural ceramic has a controllable structure and higher porosity and bending strength through a photo-curing reaction and reaction sintering of the photoinitiator in a layer-by-layer stacking and forming manner. The ceramic has the porosity being 71.1%-77.4% and the bending strength being 72.6-109.3 MPa.