SiC nanofibers are a typical high-performance ceramic material, which have attracted significant attention in high-tech fields such as aviation, aerospace, and military equipment due to their excellent performance under extreme conditions. The microwave heating process involves the microwave coupling effects, the microwave plasma effects, and high-temperature physicochemical reactions. Porosity is one of the primary factors influencing the thermal effects during microwave heating. Porous green bodies were prepared using SiO₂ sol and carbon powder as raw materials, combined with the direct foaming method and the emulsion templating method for pore formation, along with in-situ curing technology for SiO₂ sol-gel. By varying the quality of the raw materials and the oil-to-water ratio, porous green bodies with different pore sizes and porosity were prepared. By varying the mass, volume, and pore size of the green bodies during the sintering process, we aim to investigate how changes in the pore structure of porous green bodies affect the microwave thermal effects and to elucidate the growth mechanism of the spatial network of SiC nanofibers. The results show that when the volume of the green bodies is controlled, the resulting three-dimensional network of SiC nanofibers exhibits a fine, straight, and elongated morphology, with lengths ranging from 80 to 100 μm and diameters ranging from 50 to 100 nm. As the mass increases, the SiC nanofibers become straighter. As the mass of the green bodies remains unchanged, the reduction in volume results in SiC fibers with a finer and longer network-like morphology. Under conditions of the same mass and volume, the larger the pore size of the porous green bodies, the more slender the morphology of the synthesized 3D network of SiC nanofibers. In this study, by subjecting porous green bodies to microwave heating, we found that the presence of a porous structure effectively stimulates microwave plasma phenomena during the heating process, thereby enabling the synthesis of 3D network-like SiC nanofibers.