An optical engine includes a light source system for providing an incident light beam, a reflective light valve, a first case, a projection lens and a second case. The reflective light valve is disposed in the first case for receiving the incident light beam, reflecting and outputting an image light beam or a dumped light beam. The first case provides a first opening located in the light path of the dumped light beam. The projection lens is connected to the first case for receiving and projecting the image light beam to form an image. The second case intercommunicates with the first case via the first opening. The dumped light beam enters the second case through the first opening, and is reflected twice or more inside the second case. Moreover, the inner wall of the second case applies to absorb the energy of the dumped light beam.