The biological function of a protein depends not only on the correct primary amino acid sequence, but also on achieving its native three-dimensional structure. Thus, correct folding of a protein is of great significance to life activities. Due to the complex and crowded intracellular environment, the folding of many proteins is often difficult in vivo. One category of proteins, called chaperones, help other proteins to fold correctly. Chaperones can recognize and stabilize other instable protein to assist its folding. Recent studies showed that, the ring-shaped chaperone GroEL can repetitively unfolding kinetically trapped protein folding intermediate, giving the intermediate another chance to refold, thus increases its overall folding rate. The detailed mechanism of GroEL assisted folding is still under controversy. In this review, we briefly summarize the recent progress in the study of the latter mechanism.