The crack growth mechanism of rubber composites has not been explored clearly. Here, we focus on the microstructure evolution of styrene-butadiene rubber/silica (SBR/silica) composites during crack propagation. Two groups of SBR/silica composites are constructed. The silica nanoparticles used in the two groups have different sizes but the same surface properties. By crack propagation and small angle X-ray scattering measurements, the crack growth rate, nanocavitation and silica distribution morphology of the rubber composites are investigated. The role of damage structure nucleation and growth during crack propagation of the SBR/silica composites is discussed. It is demonstrated that the damage structure nucleation is the key factor for the crack growth at large tear energies, while the growth feature of damage structure dominates the crack growth at smaller tear energies. This work provides us with better understanding on the crack growth mechanism of rubber composites.