Abstract: The mechanical energy conservation in the two independent descending and ascending processes for a YO-YO or a rotating-wheel pendulum is pointed out,but it is false to consider that the mechanical energy stays the same at the turning point of its lowest position,because of ignoring the energy loss caused by system jump effect.This mistake has long been existed in teaching,textbooks and all kinds of other books,worthy of being brought to the forefront.

Key words: YO-YO（rotating-wheel pendulum）, mechanical energy conservation, turning point, rigid model