<p>In practical projects, concrete members are often subjected to multiple axial stress states, and the actual fracture process is more complex. This study utilizes a novel gap tensile test method and two-dimensional random circular aggregate mesoscopic numerical simulation to investigate notched concrete fracture characteristics. On the basis of Bažant's Type II size effect law (SEL) and linear elastic fracture mechanics (LEFM), fracture parameters such as fracture energy, notch tensile strength, and fracture toughness of concrete under biaxial tensile stress were obtained. The results indicate, compared with uniaxial loading, that crack–parallel tensile stress significantly affects the key fracture parameters of concrete. As the crack–parallel tensile stress increases, the peak fracture load of all the concrete samples tend to monotonically decrease. Compared with that when the normalized crack–parallel tensile stress η = 0, the peak load decreases by approximately 5, 20, and 40% when η is 0.19, 0.38, and 0.57, respectively, for different specimen dimensions. When the normalized crack–parallel stress increases to 0.57, the fracture energy, fracture toughness, and notch tensile strength decrease by approximately 30–80%. The crack–parallel tensile stress induces predamage in the weak interfacial transition zone (ITZ) of the concrete, leading to a reduction in the load-bearing capacity. Likewise, it can be expected that this degradation effect will be even more severe in low-strength concrete.</p>