Bending fatigue is the main failure form of gear transmission, which seriously affects the service performance of gears. Shot peening strengthening technology is a common surface strengthening method for gears that can effectively improve the bending fatigue resistance for gear roots. Two typical shot peening strengthening methods (air-blast peening and centrifugal peening) were introduced, which were compared from the aspects of application range, machining efficiency, spraying principle, surface integrity, etc., respectively. To reveal the mechanism of shot peening strengthening, numerical simulation and parameter optimization methods of shot peening were summarized, influence mechanism of shot peening on the surface integrity of gear roots was analyzed, and correlation of surface integrity parameters was expounded. Meanwhile, the fatigue fracture mechanism (including crack initiation and propagation) of shot-peened gears were revealed. On this basis, influence factors of bending fatigue for shot-peened gears were emphatically discussed, and measures to improve the fatigue strength of gears were put forward. In addition, the gain in fatigue strength of shot-peened gear was discussed briefly, and bending fatigue modeling methods for shot-peened gears were generalized. Finally, shot-peening strengthening process of gear roots was summarized and its development trend was prospected.