1. An imaging method for an image sensor, wherein the image sensor comprises a photosensitive pixel array and a filter arranged on the photosensitive pixel array, the filter comprises an array of filter units, each filter unit and 2 * 2 adjacent photosensitive pixels covered by the filter unit in the photosensitive pixel array form a synthesized pixel, four adjacent synthesized pixels corporately form a synthesized pixel unit, and four filter units arranged adjacently within each synthesized pixel unit comprise a red filter unit, a blue filter unit and two green filter units, each photosensitive pixel comprises a photodiode, one photosensitive pixel corresponds to one source follower and one analog-to-digital converter, and the method comprising: reading an output of the photosensitive pixel array, and extracting pixel values of photosensitive pixels located at same positions or different locations in four adjacent synthesized pixels of the synthesized pixel unit from a read-out single-frame high-resolution image, and combining the pixel values so as to obtain four frames of low-resolution images; and synthesizing the four frames of low-resolution images.