An optical device is capable of controlling a projector, wherein the projector is capable of wirelessly receiving an image signal from a data source, combining an identification signal into the image signal to generate a projection signal, and projecting a projection image according to the projection signal. The optical device includes an optical sensor. The optical sensor is capable of sensing the identification signal of the projection image, outputting a detection signal corresponding to the identification signal and wirelessly transmitting the detection signal to the projector. A projection system including a projector and an optical sensor is provided as well.