The disclosure provides a projector, an image generation device, and an image generation method thereof. The projector includes an image generator, a light combiner, and a projection lens. The image generator includes light valves and generates an image beam. The light combiner includes a diffusing and filtering component, which generates a combined beam, and the image generator receives the combined beam from the diffusing and filtering component. In a first time period, the light combiner converts a first input beam into partial beams. The partial beams are transmitted to the light valves respectively. In a second time period, the light combiner receives a second input beam. The diffusing and filtering component filters out one of the partial beams, and the other of the partial beams are transmitted with the second input beam respectively to the light valves.