Abstract: An auto-focus projection system includes a light valve, a projecting lens, a camera module, a processing circuitry, and a focusing adjustment assembly. The light valve converts an illumination beam into an image beam, and the projection lens projects the image beam. The projection lens forms different images with respective sharpness values at different focusing positions The camera module captures the different images and converting the different images into electrical signals. The processing circuitry receives the electrical signals and compares the sharpness values to generate a control signal. The focus adjustment assembly receives the control signal and moves the projection lens according to the control signal.