This disclosure provides an image composition system and its method. The system includes: an image capturing unit configured for capturing at least one first image of a real object; a depth information generating unit disposed on the image capturing unit and configured for measuring a depth information between the depth information generating unit and the real object; a computing unit receiving the at least one first image and the depth information, performing the computation of removing a noise caused by a moving object other than the real object from the received depth information, and computing a moving trajectory of the image capturing unit; and a composition unit compositing the at least one first image and a second image of a virtual scene, and display the composited result on a display unit.