A folding elliptical exercise machine includes a main frame, a guider frame coupled to the main frame for being moved between a use position and a storage position, a rotatable crank mechanism arranged on the main frame, two supporting members coupled between two crank units of the rotatable crank mechanism and two tracks of the guider frame, and two pedals coupled to the two supporting members. The engagement of the locking device is affected by the position of the guider frame. The locking device has a latch which can be located in an unlocked position where it is not interfering with the rotatable crank mechanism when the guider frame is oriented in the use position, or can be located in a locking position for stopping the rotatable crank mechanism from rotating when the guider frame is moved from the use position to the storage position.