A projection system includes a projector, an uncalibrated DDL output unit, a light detection device, a characteristic curve computation unit, a calibration unit and a calibrated DDL output unit. The uncalibrated DDL output unit is used for driving the projector to project a frame to an area of a projected object. The light detection device is used for detecting brightness values of the area. The characteristic curve computation unit is used for obtaining a characteristic curve based on the uncalibrated DDLs and the detected brightness values. The calibration unit is used for executing a calibration process according to the characteristic curve and GSDF, so as to map each of the uncalibrated DDLs to a calibrated DDL. The calibrated DDL output unit is used for driving the projector to project another frame to the area of the projected object.