A projection system and a projected image stacking method are provided, in the method, an individual representative position of each captured image is identified, wherein the captured images are obtained by respectively capturing an area projected by multiple projectors, and each individual representative position is related to image content projected by each projector. A common representative position is set in the area. A projection lens of each projector is shifted according to a distance difference between each individual representative position and the common representative position. In this way, the projection lens is quickly shifted, and is further applied to stacking operations. The operation of multiple projectors is as convenient as the operation of a single projector based on control of a remoter or keys of a projector control interface.