The invention discloses a generator injection type stator grounding protection method and a protection device. The method includes the following steps that: low-frequency voltage at two ends of an external power source and low-frequency current which flows through the external power source are detected, and the internal resistance value of the injection power source device as well as the load resistance value and leakage impedance value of a grounding transformer are obtained respectively; based on low-frequency alternating-current loop phasor calculation, a generator stator winding grounding fault transition resistance value Rg is obtained according to an admittance method; when the detected Rg is lower than a protection alarm value or a tripping value, a protection action is an alarm action or a tripping action; or an input impedance amplitude value is obtained according to the low-frequency voltage and the low-frequency current, and when the detected input impedance amplitude value is lower than a set high constant value of the protection device, the protection action is an alarm action; and when the detected input impedance amplitude value is lower than a set low constant value of the protection device, and the protection action is a tripping action. The method is suitable for a mode in which low-frequency signals are injected from a generator neutral point grounding transformer and is also suitable for a mode in which low-frequency signals are injected from a machine-end open-delta PT.