In this study, a low-cost bicycle power assistance device, which can operate without altering the frame structure of the bicycle, is proposed. It does not require disassembling the tires or the wheel frame, making it an easily installable add-on system. Compared with other existing electric-assist bicycles or retrofit kits, the scalability and hardware compatibility of the device developed in this study are relatively higher. The system primarily relies on a microcontroller unit (MCU) core, with HT66F2390 as the main controller, integrating a brushless electric motor for power drive, a Hall sensor to detect sprocket speed, MCU switches, a magnet disk, and a 350 W lithium battery. First, without modifying the bicycle frame, a fixture for securing the power-assist motor is designed to attach the motor to the bottom edge of the frame. In this study, we recorded the power of several riders traveling the same approximately 5 km route twice, before and after modification. Before modification, the one-way riding power was 200 W, and after modification, it increased to 230 W, representing an increase of approximately 20%. These data indicate that the system assists riders in conserving energy.