A method for adjusting output images and multi-pictures display system are provided. The method includes following steps. A monitor resolution, a multi-pictures display mode and a rotation status of a display device are obtained. A display setting in an operating system (OS) of an image source apparatus is obtained. Whether the display setting in the OS satisfies an optimal display condition is determined according to the monitor resolution, the multi-pictures display mode and the rotation status of the display device. If the display setting does not satisfy the optimal display condition, the display setting in the OS is adjusting according to the optimal display condition. If the display setting satisfies the optimal display condition, a plurality of pixels of a single frame output by the image source device is one-by-one corresponding to a plurality of pixel units of the display device configured for displaying the single frame.