Concrete is regarded as a mesoscopic heterogeneous material consisting of aggregate, mortar, interfacial transition zone and pore defects. A concrete mesoscopic model considering the pore defect phase is established using computed tomography(CT) technology. The tensile failure process and stress concentration of concrete with and without pore defects are compared and analyzed, and the area distribution of pore defects was statistically studied. The results show that the existence of pore defects can enhance the overall heterogeneity of concrete and make the tensile stress distribution more uneven, resulting in more significant stress concentration when considering pore defects in concrete. The size distribution of pore defects conforms to the Lognormal probability distribution.