A projector includes a case, a projection lens, a shielding plate, and a controller received in the case. The case includes a sidewall defining an aperture. The projection lens is used for projecting a cone of light rays which passes through the aperture and a projection area. The shield plate is positioned on the sidewall. The driver is used for driving the shield plate to any of desired positions ranging from a first position in which the shield plate exposes the aperture to a second position in which the shield plate shields the aperture. The barrier detection device is used for generating a first signal if detecting no barrier presents in the projection area and generating a second signal if detecting a barrier presents in the projection area. The controller is used for controlling the driver to drive the shield plate to the first position if receiving the first signal.