A pedal power generating device includes a base having two supporting arms upwardly extending therefrom. The base has a gearbox mounted thereon. A pedal assembly is mounted between the two supporting arms. The pedal assembly has a pedal, a supporter extending from the pedal, a connecting rod assembled with the supporter and connected to the gearbox, and a twist spring mounted between the pedal and the two supporting arms. A transmitting assembly is mounted in the gearbox. The transmitting assembly includes a driver, a first gear train engaged with the driver, a second gear train engaged with the first gear train, and a steering gear set simultaneously engaging with the first gear train and the second gear train. A generator is mounted adjacent to the gearbox, and the generator has a rotor engaged with the second gear train.