The Positive-continous-engagement（ PCE） overrunning clutch is the key component in helicopter transmission system. With wearing happened both in the high-speed differential speed and gripping process that lead to the change of the working condition of PCE overrunning clutch. The sprag flip and the sprag pop-in and out caused by wearing is the main failure mode of the clutch. The grip friction angle condition is derived,by establishing the dynamic model of the clutch,the reason of the sprag pop-in and out phenomenon and its influences on the clutch is analyzed. Based on the result,both the excess of initial engage angle and the increase of the engage angle caused by clutch wear will lead to the sprag Pop-in and out. The result could be used in the fault diagnosis of helicopter transmission system.