Because cis-polybutadiene rubber (BR) can improve the fatigue crack growth (FCG) resistance of natural rubber (NR) in the low-tearing-energy (G) range, a blending system of NR/BR is often used in tire materials. In this study, based on relationship of (FCG rate (dc/dN) − G) established from the perspective of fracture mechanics and then inspired by Persson's theory and starting from the perspective of viscoelasticity, the FCG behavior and mechanism of NR/BR were investigated. NR/BR with different blending ratios were prepared, and dc/dN under different G inputs (500/1500 J/m2) was measured. According to the viscoelastic parameters (storage modulus E′, loss factor tan δ, and loss compliance modulus J″) recorded in situ, energy dissipation distribution at the crack tip (energy consumption for FCG inside crack tip: G0 and energy loss in linear viscoelastic zone near crack tip: G0f [v, T]) was determined, and the relationship of (viscoelasticity − dc/dN) was finally set up. When G = 500 J/m2, blending BR can reduce dc/dN as compared with pure NR. On one hand, with a higher BR fraction, an increased cross-linking density and enhanced filler network provided greater rigidity, which increased E′; on the other hand, a low glass transition temperature and flexible chain of BR reduced hysteresis, which decreased tan δ. The joint action led to a decrease in J″, which caused more G0f (v, T) and less G0, resulting in the ultimate reduction of dc/dN. In contrast, for G = 1500 J/m2, when the BR content was >50 phr, dc/dN showed a significant increase. Although more BR evidently decreased J″ and then led to a large amount of G0f (v, T), due to absence of strain-induced crystallization, the chain orientation of BR was hard to resist FCG when G increased. Finally, the morphology of the crack tip propagation path was captured to corroborate the different orientation characteristics of NR and BR and their effects on FCG behavior.