Abstract: Porosity is one of the important characteristics of rock mass. It can reflect the effect of weathering and altering for the physical and mechanical characteristics of rock mass. Therefore, the functional relationship of porosity and the other physical and mechanical parameters is established through indoor experiments. The effect of weathering and altering for the physical and mechanical characteristics of granite-porphyry also is analyzed. The results show that the dry density and compressive strength of the weathered and altered granite-porphyry decrease as porosity increases, which presents the power function relationship. Their correlation coefficient are greater than 0.83.The water absorption increases with increasing in porosity, which presents the binomial functional relationship. Their correlation coefficient are greater than 0.90.All the weathering and altering can weaken the physical and mechanical characteristics of granite-porphyry. As increasing in weathering and altering, the physical and mechanical characteristics of granite-porphyry can gradually reduce. The influence of altering and weathering for the steep-inclined structural face is stronger than for the gently dipping structural face.