1. A method for composing a three dimensional image with a long focal length, applicable to a three dimensional imaging system comprising a zoom lens and a fixed focal lens, wherein the fixed focal lens is provided with a first focal length, the method comprising: adjusting a focal length of the zoom lens to the first focal length, and capturing a first image and a second image with the zoom lens and the fixed focal lens respectively; adjusting the focal length of the zoom lens to a second focal length, and capturing a third image with the zoom lens; positioning a region occupied by the third image in the first image according to a magnifying power of the second focal length with respect to the first focal length; calculating a first disparity map between the first image and the second image in the region; magnifying the first disparity map according to the magnifying power to obtain a second disparity map; and composing a fourth image by using the third image and the second disparity map to simulate the image captured by the fixed focal lens with the second focal length, and outputting the third image and the fourth image as a three dimensional image.