An optical motion detecting device for a flight vehicle includes a base, an optical motion sensor and an operating processor. The optical motion sensor is disposed on the base and adapted to capture a plurality of frames. The operating processor is electrically connected with the optical motion sensor. The operating processor analyzes a pattern within the plurality of frames to acquire displacement of the base relative to a reference plane according to a known height value, and the known height value represents a height that the flight vehicle starts to move.