The present disclosure illustrates a work mode control method. The work mode control method includes the following steps. First, capturing an image once every capturing interval. Next, determining a shutter time, an average brightness and an image quality in response to the images. The shutter time is associated with a frequency of the optical encoder to capture the images. The average brightness is associated with a brightness of environmental light sensed by the optical encoder. The image quality is associated with clarity of the images. Then, adjusting a work mode of the optical encoder in response to the shutter time, the average brightness and the image quality. Finally, controlling the optical encoder to enter a rest mode when the shutter time is higher than a first threshold, the average brightness is higher than a second threshold, and the image quality is lower than a third threshold.