This paper systematically reviews the research progress on gear fatigue life, focusing on theoretical, simulation, and experimental approaches. Theoretically, mature standards (e.g., ISO 6336) provide foundational guidance for gear strength verification, while advanced models based on damage mechanics and strain-life methods offer deeper insights into bending and contact fatigue mechanisms. Simulation techniques, utilizing finite element analysis and multi-body dynamics, enable precise characterization of stress distribution and crack propagation, facilitating effective fatigue life prediction and design optimization. Experimentally, fatigue test rigs remain indispensable for validating material performance, surface treatments, and lubrication effects, though their application is often limited by time and cost constraints. Moving forward, research trends emphasize the development of efficient multi-field coupling models and the integration of theory, simulation, and testing to enhance prediction accuracy and gear reliability. This review serves as a comprehensive reference for anti-fatigue design and future studies in gear transmission systems.