In this study, by reproducing flowfields of several kinds of cavitation instabilities by numerical simulation of unsteady cavitation in three blades cyclic cascade, occurrence mechanism of rotating cavitations in cascade are attempted to clarify. "The order of appearance of rotating cavitations" and "the discontinuity of the propagation speed at the transition" are still not clarified. From the present study, it is found that they can be explained by existence of latent rotating-stall and decrease of frequency of cavity break-off with development of the cavity. And our numerical results agree with the predicted rotating speed of rotating cavitation and break-off frequency of cavitation. Additionally, difference of sub-synchronous rotating cavitation and rotating-stall cavitation, they are both sub-synchronous phenomenon but have different propagation speed, is explained according to their occurrence mechanism.