There are disclosed a porous composite comprising silicon-substituted hydroxyapatite and .beta.-tricalcium phosphate (.beta.-TCP), and a method for preparing the same. The porous composite is prepared by subjecting natural coral to hydrothermal and solvothermal reactions to prepare silicon-substituted hydroxyapatite (Si-HA) and subjecting the Si-HA to a heat treatment process. Thereafter, Si-HA and .beta.-TCP may be mixed in the porous composite. As a result, the porous composite is excellent in biocompatibility and biodegradability. Also, the porous composite functions to maintain the microstructure of coral and is similar to natural bone in terms of composition and shape. Accordingly, the porous composite may be effectively used as a bone tissue repairing material and a bone graft material that can substitute for human hard tissue.