An image sensor includes P (P.gtoreq.2) red-light, P green-light and P blue-light photosensors and is mounted in an image scanning device to expose and read multiple scan lines of a to-be-scanned document. The scanning device includes a driving device for driving the image sensor to move relative to the document. When the image sensor acquires an image of the document, the P red-light photosensors, the P green-light photosensors and the P blue-light photosensors read 3P adjacent scan lines respectively in each exposure. The image sensor continues to read the 3P corresponding adjacent scan lines when the driving device further drives the image sensor to move by P scan lines relative to the document in a specific direction in each step of the driving device. The driving device continues to drive the image sensor to move relative to the document until all the scan lines have been read.