The invention discloses a fatigue life evaluating device for flexible gears. The fatigue life evaluating device comprises a mechanism bottom plate and a transmission shaft arranged horizontally and used for mounting a tested flexible gear. A fixing support, a loading device, a driving device, a magnetic powder brake, a torque sensor and a speed increaser are mounted on the mechanism bottom plate, the fixing support is used for fixing the flexible gear and mounting the transmission shaft, the loading device is fittingly installed with the fixing support and is used for loading the tested flexible gear, components of the driving device are sequentially and fittingly connected to the tested flexible gear, and the magnetic powder brake, the torque sensor and the speed increaser are installed according to the preset requirement on coaxiality and are sequentially and fittingly connected to the tested flexible gear. The driving device and the loading device are fittingly installed according to the preset requirement on coaxiality, and the loading device and the speed increaser are fittingly installed according to the preset requirement on parallelism and the preset requirement on a meshing clearance of gears. The fatigue life evaluating device for flexible gears can overcome shortcomings that an operation process is complicated, time and labor are wasted, cost is high, testing effects are unsatisfactory, and the like in the prior art. Besides, the fatigue life evaluating device has the advantages of simplicity and convenience in operation process, capability of saving time and labor, low cost and perfect testing effect.