A projector, a projection system and an image projection method are provided. The projector includes an image processing circuit, a processor circuit and a memory circuit. The image processing circuit is configured to adjust a first image. The processor circuit is electrically connected to the image processing circuit. The processor circuit is configured to combine the first image with a second image in a first display mode of the projector. The memory circuit is electrically connected to the processor circuit. The memory circuit is configured to store the second image. The projector projects a display image formed by combining the first image and the second image to a projection target.