A method for calibrating a projection image to be displayed by a projection system, wherein the projection system comprises a projection device and a handheld electronic device, the method for calibrating the projection image comprising: setting, by the handheld electronic device, a correction image having more than three identification patterns (IDP1-IDP4); establishing, by the handheld electronic device, a first coordinate system associated with the correction image; calculating, by the handheld electronic device, first coordinate information of the identification patterns (IDP1-IDP4) in the first coordinate system, wherein the first coordinate information comprises coordinate values of geometric centers (C1-C4) of the identification patterns (IDP1-IDP4) in the correction image; driving, by the handheld electronic device, the projection device to automatically project the correction image when executing an application built in the handheld electronic device; projecting the correction image on a display screen (410, 520) by using the projection device; performing an image capturing operation on the correction image and the display screen to obtain a captured image comprising captured identification patterns (IDPA1-IDPA4) by an image capturing device configured in the handheld electronic device; establishing, by the handheld electronic device, a second coordinate system associated with the captured image; calculating, by the handheld electronic device, second coordinate information of the captured identification patterns (IDPA1-IDPA4) in the second coordinate system, wherein the second coordinate information comprises coordinate values of geometric centers (CA1-CA4) of the captured identification patterns (IDPA1-IDPA4) in the captured image; calculating, by the handheld electronic device, coordinate translation information according to the first coordinate information and the second coordinate information; calculating, by the handheld electronic device, boundary coordinate values of a plurality of corners (DPC1-DPC4) of the display screen (410, 520) in the captured image in the second coordinate system; translating, by the handheld electronic device, the boundary coordinate values according to the coordinate translation information to obtain a plurality of translation coordinate values of the corners (DPC1-DPC4) of the display screen (410, 520) in the first coordinate system, and to obtain boundary information of the display screen in the first coordinate system; and scaling, by the projection device, the projection image to obtain an adjusted projection image aligned with the display screen (410, 520) according to the boundary information of the display screen in the first coordinate system. setting, by the handheld electronic device, a correction image having more than three identification patterns (IDP1-IDP4); establishing, by the handheld electronic device, a first coordinate system associated with the correction image; calculating, by the handheld electronic device, first coordinate information of the identification patterns (IDP1-IDP4) in the first coordinate system, wherein the first coordinate information comprises coordinate values of geometric centers (C1-C4) of the identification patterns (IDP1-IDP4) in the correction image; driving, by the handheld electronic device, the projection device to automatically project the correction image when executing an application built in the handheld electronic device; projecting the correction image on a display screen (410, 520) by using the projection device; performing an image capturing operation on the correction image and the display screen to obtain a captured image comprising captured identification patterns (IDPA1-IDPA4) by an image capturing device configured in the handheld electronic device; establishing, by the handheld electronic device, a second coordinate system associated with the captured image; calculating, by the handheld electronic device, second coordinate information of the captured identification patterns (IDPA1-IDPA4) in the second coordinate system, wherein the second coordinate information comprises coordinate values of geometric centers (CA1-CA4) of the captured identification patterns (IDPA1-IDPA4) in the captured image; calculating, by the handheld electronic device, coordinate translation information according to the first coordinate information and the second coordinate information; calculating, by the handheld electronic device, boundary coordinate values of a plurality of corners (DPC1-DPC4) of the display screen (410, 520) in the captured image in the second coordinate system; translating, by the handheld electronic device, the boundary coordinate values according to the coordinate translation information to obtain a plurality of translation coordinate values of the corners (DPC1-DPC4) of the display screen (410, 520) in the first coordinate system, and to obtain boundary information of the display screen in the first coordinate system; and scaling, by the projection device, the projection image to obtain an adjusted projection image aligned with the display screen (410, 520) according to the boundary information of the display screen in the first coordinate system.