A projection system and a method for calibrating display image are provided. The method includes: setting a correction image having identification patterns, and establishing first coordinate information of the identification patterns; using a projection device to project the correction image on a display screen; performing an image capturing operation to the correction image and the display screen to obtain a captured image; calculating second coordinate information of the identification patterns in the captured image, and calculating coordinate translation information according to the first coordinate information and the second coordinate information; calculating boundary information of the display screen according to a plurality of boundary coordinate values of the display screen in the captured image and the coordinate translation information, such that the projection device adjusts a size of a projection image for corresponding to the display screen according to the boundary information.