Based on the theory of Double-K fracture model, and by using the standard three-point bending concrete specimens, the initial crack load Pini, fracture load Pmax and critical mouth open displacement (CMOD) of five specimens with relative notch depth a0/h of 0-0.6 were measured in the experiment, and the initial fracture toughness KIcini and unstable fracture toughness KIcun were also calculated. The crack tip and the fracture surface were observed by using a stereomicroscope to investigate the crack development, and the influence of the relative notch depth on the fracture performance of the concrete was analyzed. The results indicate that the larger the relative notch depth is, the earlier the specimen starts cracking, but the ratio of the initial crack load to the fracture load does not change with the increase of the initial seam height a0: the initial seam height is linearly related to the effect of initial crack load and fracture load. The initial fracture toughness and unstable fracture toughness did not decrease with the increase of the relative notch depth. The ratio of the two had no obvious change with the change of the relative notch depth, that is, the double-K fracture parameter depends on the inherent characteristics of the material itself and irrelevant to the relative notch depth. The fracture process of concrete is complicated, and there is a significant subcritical expansion stage. The subcritical crack growth appeared a kink path, by passing the aggregate in the process of development.