In this study, an interfacial spring element model with random defects was constructed based on the four-phase material model of concrete, which consists of aggregate, matrix, interfacial transition zone (ITZ), and initial defects distributing randomly within the ITZ. The ITZ is modeled by linear spring element of zero length. Simulation of concrete strength under uniaxial tension yielded numerical results that are consistent with reliable experimental data, thus proving that the model is reasonable and applicable. Then, the model was employed to analyze the influence of random initial defects on concrete strength. Results showed that increased initial defects in ITZ leads to a linear decrease in tensile strength of the concrete. Compared with the strength of concrete without initial defects, that of concrete with initial defects equal to 60 % of ITZ elements decreased by about 40%. Based on these results, this study recommends that the fraction of initial defects should be 30 % when using the proposed model to predict uniaxial tensile strength of normal-strength concrete.