Joining technology has become an important part of the preparation and engineering application of large or complex-shaped C_f/SiC composites. In this study, the stable joining of C_f/SiC composites was achieved via the Si-C reaction joining method by using the phenolic resin as the carbon precursor. Effects of the bulk density and pore size of porous carbon blanks on the joining properties and microstructure of the joints were investigated. Effect of the content of inert fillers on the joining properties and microstructure of joint were discussed. Bulk density and pore size of the resin-based porous carbon blank are suitable to be set in the range of 0.71-0.90 g·cm^-3 and 200-600 nm, respectively. The size of free silicon increases gradually with the increase of the pore size. The flexural strength of the joined specimens can reach (125±12) MPa at pore size of 190 nm. Addition of SiC inert filler is obviously beneficial to reduce the volume shrinkage of the porous carbon blank. Flexural strength of the joined specimens reached the highest value, i.e., (216±44) MPa at the inert filler content of 50%. Overall, this study provides theoretical guidance for the stable joining of C_f/SiC composites, which has significance for realizing the preparation and engineering application of complex-shaped or large C_f/SiC composites.