A power generation system used on a vehicle which includes a drive pedal operable by a driver to make a drive member to generate corresponding rotation speed change so that the vehicle is moving at a forward speed and a brake pedal operable by the driver to activate a tire brake to provide a deceleration force to a tire of the vehicle, whereby: a first rotary member which is run through by a wheel axle and includes a first gear with a first number of teeth, an electromagnetic winding and a housing space corresponding to the wheel axle, and is driven by spinning of the wheel axle to form a first rotation displacement; a second rotary member which is located in the housing space and on the wheel axle coaxial with the frst rotary member, and includes a second gear with a second number of teeth, an excitation winding corresponding to the electromagnetic winding and an electric input portion connected to the excitation winding; a transmission gear set which is a planetary gear transmission structure formed by the first rotary member and the second rotary member, the transmission gear set includes a support rack fixedly located on the vehicle and at least one transmission gear hinged on the support rack to engage with the first gear and the second gear; each transmission gear driving the second rotary member during the first rotation displacement to generate a second rotation displacement in a direction opposite to the first rotation displacement so that the electromagnetic winding and the excitation winding generate opposite movements; and a brake module which is connected to the brake pedal and the drive pedal and includes a first brush corresponding to the electric input portion, and detects operation of the brake pedal by the driver or operation suspension of the drive pedal by the driver so that an excitation current is output through the first brush to the electric input portion to make the excitation winding to generate excitation to produce magnetic cutting with the electromagnetic winding to generate electric power, characterized in that the second number of teeth is smaller than the first number of teeth. a first rotary member which is run through by a wheel axle and includes a first gear with a first number of teeth, an electromagnetic winding and a housing space corresponding to the wheel axle, and is driven by spinning of the wheel axle to form a first rotation displacement; a second rotary member which is located in the housing space and on the wheel axle coaxial with the frst rotary member, and includes a second gear with a second number of teeth, an excitation winding corresponding to the electromagnetic winding and an electric input portion connected to the excitation winding; a transmission gear set which is a planetary gear transmission structure formed by the first rotary member and the second rotary member, the transmission gear set includes a support rack fixedly located on the vehicle and at least one transmission gear hinged on the support rack to engage with the first gear and the second gear; each transmission gear driving the second rotary member during the first rotation displacement to generate a second rotation displacement in a direction opposite to the first rotation displacement so that the electromagnetic winding and the excitation winding generate opposite movements; and a brake module which is connected to the brake pedal and the drive pedal and includes a first brush corresponding to the electric input portion, and detects operation of the brake pedal by the driver or operation suspension of the drive pedal by the driver so that an excitation current is output through the first brush to the electric input portion to make the excitation winding to generate excitation to produce magnetic cutting with the electromagnetic winding to generate electric power, characterized in that the second number of teeth is smaller than the first number of teeth.