Knee osteoarthritis (OA) is a highly prevalent and disabling disease that causes pain and gradual degeneration of the articular cartilage. Phototherapy is a new physiotherapy treatment, more effective and stable than other non-pharmacologic management. Conventional phototherapy devices typically suffer from unintelligent and bulky equipment, while existing phototherapy methods require maintain a certain phototherapy distance. Here, we introduce a wearable flexible phototherapy device worn on a knee for osteoarthritis, incorporating a phototherapy adhesive patch and a control box. The phototherapy adhesive patch is capable of softly laminating onto the curved surfaces of the knee skin to increase the effects of phototherapy. We describe the LED array, constant current drive module, key control module, and power supply module that serve as the foundations for the control box. The weight of the device is only 101.8 g. The irradiance of the device can be adjusted linearly and irradiance of the designed phototherapy device based on LED can reach 13 mW/cm². The maximum temperature of the surface of the light source is 31.2 °C. The device proposed in this work exhibits satisfactory stability, promising a potential in phototherapy.