A projection system and a self-adaptive adjustment method are provided. The projection system includes a projection device, an image capturing device, and a processing device. The projection device sequentially projects sub-pattern arrays of a pattern array to a projection region of a projection surface. The image capturing device sequentially captures the sub-pattern arrays projected on the projection surface to output pattern images. The processing device analyzes the pattern images to obtain pattern coordinates and a pattern order of projected patterns in each pattern image. The processing device determines at least a part of the projected patterns to be effective patterns according to the pattern coordinates and the pattern order, and adjusts the projection device according to the effective patterns. The projection system and the self-adaptive adjustment method provide good projection quality.