A computer system and a power-management method thereof are provided. The computer system has an image-reading mode, a first power-management mode and a second power-management mode, and the computer system operating in the second power-management mode consumes less power than it consumes in the first power-management mode. The computer system comprises a first portion comprising a graphics processing unit, a memory space and a display; and a second portion comprising a storage storing an image data. When the computer system operates in the image-reading mode, the image data has been transferred to the memory space from the storage, the second portion enters to the second power-management mode from the first power-management mode, and the first portion keeps in the first power-management mode, so that the graphics processing unit can display an image by the display according to the image data stored in the memory space.