The invention provides a portable computer system for capturing a target image related to an item. The portable computer system comprises a base, a cover, an image capturing module, and a lightness adjusting module. The cover is pivotally connected to the base, and comprises a panel. According to the size of the item, the cover rotates with respect to the base to project the light radiated from the panel onto the base. The image capturing module is used for rotating with respect to the cover to capture a reference image according to the size of the item. The lightness adjusting module is used for adjusting the lightness of the panel according to the reference image. After the lightness of the panel is adjusted, the item is deposed on the base, and the image capturing module captures the target image related to the item.