The invention discloses a projection image processing method and device and a projection display system. Resolution adjustment is performed on a first image of the first resolution under projection so that a second image of the second resolution is obtained, wherein the pixel number of the second resolution in the first direction is N times of the pixel number of the resolution of a light valve in the first direction, the pixel number of the second resolution in the second direction is not greater than the pixel number of the resolution of the light valve in the second direction, and the second image and the first image are identical in the information amount level; and the second image is divided into N frames of images of which the resolution is not greater than that of the light valve, and the N frames of images are displayed in a time sharing way. Loss of the original pixel information amount of the high-resolution image can be avoided.