In this study, the stochastic characteristics of the fracture process zone (FPZ) of concrete are analysed. The impacts of specimen size on the critical crack opening displacement, the distribution of cohesive stress, and the critical length of FPZ are studied, and calculations are performed for the stochastic energy consumption curves of concrete. The dispersion of the variance of critical crack tip opening displacement CTODc and the critical length of FPZ are obtained. Combined with the existing experiments, comparative verification is performed. The results show that the critical crack tip opening displacement CTODc and critical length of FPZ increased with the specimen size, whereas the critical cohesive stress at the tip of the initial crack decreased with the specimen size. The dispersion of the variance of critical crack tip opening displacement CTODc and the critical length of FPZ increased with the specimen size.