An understanding of protein folding/unfolding processes has important implications for all biological processes, in-cluding protein degradation, protein translocation, aging, and diseases. All-atom molecular dynamics (MD) simulations are uniquely suitable for it because of their atomic level resolution and accuracy. However, limited by computational ca-pabilities, nowadays even for small and fast-folding proteins, all-atom MD simulations of protein folding still presents a great challenge. An alteative way is to study unfolding process using MD simulations at high temperature. High temper-ature provides more energy to overcome energetic barriers to unfolding, and information obtained from studying unfolding can shed light on the mechanism of folding. In the present study, a 1000-ns MD simulation at high temperature (500 K) was performed to investigate the unfolding process of a small protein, chicken villin headpiece (HP-35). To infer the folding mechanism, a Markov state model was also built from our simulation, which maps out six macrostates during the folding/unfolding process as well as critical transitions between them, revealing the folding mechanism unambiguously.