Abstract: Concrete and rock are typical porous mediums which have a great influence on the elastic modulus, strength and other mechanical parameters of concrete material. The effective bulk-modulus of a composite with hollow spheres in a matrix is determined by a three-phase sphere model, and the theoretical formula of an effective shear modulus is deduced and obtained by a hollow cylindrical rod model proposed in the paper. Based on the assumption of isotropic materials, the effective Young’s modulus and Poisson’s ratio are derived. Besides, the relationships between porosity and effective tensile strength, effective compression strength and effective shear strength are deduced, and the formulas of a quantitative relation are given. Furthermore, the relation between porosity and critical strain when the material with pores reaches the effective strength is deduced. The results show that the method proposed in the paper can predict the effective mechanical properties of concrete greatly, and the formula is simple and easy to use.