The invention provides a rotary flaw-detection mechanism for a zirconium bar ultrasonic detection system. The rotary flaw-detection mechanism comprises a main shaft, conductive slip rings, probe bases, a long guide sleeve and a short guide sleeve, wherein an inner cavity of the main shaft is of a multi-cavity structure, and comprises a detection chamber, a long guide sleeve inner cavity and a short guide sleeve inner cavity; the exterior of the middle part of the main shaft is connected with five to six probe bases; the long guide sleeve is arranged at the exterior of one side of the main shaft, and the short guide sleeve is arranged at the exterior of the other side of the main shaft; the five to six conductive slip rings are arranged at the exterior of the long guide sleeve, and each conductive slip ring is correspondingly connected with the corresponding probe base; a synchronous belt wheel is arranged at the exterior of the short guide sleeve; the exterior of the long guide sleeve is provided with a water inlet, and the exterior of the short guide sleeve is provided with a water outlet; water enters the water inlet through a circulating water tank, flows into the detection chamber through a gap between the main shaft and the long guide sleeve, and fills the long guide sleeve inner cavity and the short guide sleeve inner cavity. The rotary flaw-detection mechanism has the advantages that the ultrasonic detection on zirconium alloy bars can be realized, the labor intensity is effectively decreased, and the existing problem of low detection efficiency is solved.