Protein folding/unfolding has long represented one of the considerable challenges.In this work,a total of 4.0-μs explicit solvent MD simulations were performed on protein GB1 with the gradual elongation of end-to-end distances,identifying a wide range of key intermediates and thus connecting the denatured and native states.The so obtained results agree well with the available experimental and computational reports.In addition,they provide for the first time provides many other significant folding events and a clear description of protein GB1 folding/unfolding.The folding initiates at the three turn regions.Then the three secondary structure units begin to shaping in tandem,and their folding events are intersected,whereas the hydrophobic core forms at the very late stage.The non-native contacts play a significant role at the early folding stages.Unexpectedly,the tertiary contacts occur rather early and increase concomitantly with the comprising secondary structure units; which largely determine the formation of tertiary contacts.