A projection display system and a correcting method are provided. The projection display system includes a plurality of projection devices, an image capturing device, a blending device and computing device. The projection devices are configured to project a plurality of projection regions to a projection surface. The image capturing device is configured to capture an image on the projection surface. The computing device determines locations of the projection regions according to the image captured by the imaging capture device. A display screen of the computing device is configured to display a control interface including a display screen region proportionally corresponding to the projection regions. The computing device dynamically adjusts a projection ratio of an image frame displayed on the projection region by adjusting locations of control points on the display screen region and a space between the control points.