The present disclosure provides a DC motor driving system including a DC motor, a power supply device, a switch circuit, and a microprocessor. The power supply device provides an input electrical energy. The switch circuit receives the input electrical energy and outputs a motor electrical energy, which includes a motor power and a motor voltage, to the DC motor. The DC motor driving system switchably works in a constant-voltage mode, a first variable-voltage mode, or a second variable-voltage mode. In the constant-voltage mode, the input electrical energy remains unchanged. In the first variable-voltage mode, the microprocessor controls the power supply device to adjust the input electrical energy for increasing the motor voltage and the motor power. In the second variable-voltage mode, the microprocessor controls the power supply device to adjust the input electrical energy for decreasing the motor voltage and keeping the motor power unchanged.