An image acquisition device capable of accomplishing special-effect processing on real-time motion of the image data in a preview mode is disclosed. The device includes a lens, an optical sensor, an image signal processor, a special-effect processor, a storage unit, and an image display unit. The optical sensor receives an optical signal representing an image through the lens and outputs an electric signal corresponding to the image. The image signal processor couples to the optical sensor for processing the electric signal with a first processing mode and then outputting a raw image data. The special-effect processor couples to the image signal processor for processing the raw image data with a second processing mode and then outputting a processed image data. The storage unit saves the processed image data. The image display unit displays the image based on the processed image data.