A projection device including a processor circuit is provided. A reverse-warped image is generated by the processor circuit according to an input image and a second mapping table. The projection device is adapted to project the reverse-warped image onto the projection screen. The second mapping table is converted from a first mapping table according to a plurality of warped feature points caused according to a first calibration pattern and a second calibration pattern. A projection system and an image calibration method are also provided. Therefore, an undistorted and a non-warped image may be formed on a curved projection screen and be viewed by a viewer.