A system and method for displaying a 3D image with depths, which utilize at least one light signal generator to sequentially generate multiple light signals(S100) and at least one optical assembly to receive the multiple light signals from the at least one light signal generator, and project and scan the multiple light signals within a predetermined time period to display the 3D image in space(S200). Each pixel of the 3D image is displayed at a position by at least two of the multiple light signals to a viewer's eye, paths or extensions of the paths of the at least two light signals intersects at the position and at an angle associated with a depth of the pixel, and the predetermined time period is one eighteenth of a second. Accordingly, the advantages of simplified structure, a miniatured size, and a less costly building cost can be ensured.