Abstract: A pair of projection glasses, a wearable projection apparatus, and a foldable optical engine are provided. The foldable optical engine includes a projection mechanism and a light emitting mechanism. A light input portion of the projection mechanism corresponds in position to a light output portion of the light emitting mechanism. The projection mechanism and the light emitting mechanism are pivotally connected to each other along a rotation axis so as to be rotatable relative to each other along the rotation axis. When the projection mechanism and the light emitting mechanism are rotated relative to each other to cause the light output portion to face toward the light input portion, the light emitting mechanism is configured to emit a light beam from the light output portion toward the light input portion, so as to allow the projection mechanism to receive the light beam for projecting an image light.