To probe the fatigue crack growth behavior of spur gear tooth roots, the Ansys-Franc3D co-simulation method, based on the rules of linear elastic fracture mechanics (LEFM), was utilized in study the growth mechanism of fatigue cracks at the spur gear tooth roots. The precision of the simulation calculation outcomes was verified by the findings of the gear bending fatigue test. The findings indicate that during the fatigue fracture propagation of tooth roots, the stress intensity factor (SIF) of type I significantly exceeds those of types II and III, with predominant cracking mode being opening type, complemented by sliding and tearing types. The stress intensity factor of Mode I decreases gradually from the tooth surface to the crack depth, promoting crack growth along the tooth surface. The crack at the end of the gear first extends along the depth of the crack, then along the circumference of the gear to the root.