The current invention is to disclose a de-rotating mechanism for off-axis fiber optic rotary joints. Dove prism is a de-rotating mechanism and is widely used for on-axis fiber optic rotary joints. But optic de-rotating mechanism is not available when there is a through bore along the rotational axis and optical light paths would not be allowed to path through the central area along the rotational axis. The hollow de-rotating mechanism in the current invention mainly includes a plural of fiber optic collimator patch cords, which are elaborately arranged on the circumference of said de-rotating mechanism following a special rule. The current invention eliminates the photodiodes in most prior published patents and make it possible to build a true passive, bi-directional fiber optic rotary joint without the limitation to the through bore diameters.