Ultrafast fiber lasers have become the ideal candidate for ultrafast light source and been applied in practice. Due to the small beam diameter and the long interaction distance between light and fibers, the nonlinear effect is unavoidable. Before the appearance of multiple pulses resulted from the excessive nonlinear effect, the phenomenon of period doubling bifurcation(PDB) can be observed in ultrafast fiber lasers under appropriate operation conditions. The PDB refers to the recurrence of pulse parameters at the rate of times of cavity transmission time. The PDB is the intrinsic features of nonlinear systems. It is universal among different nonlinear systems. The latest research progress of the PDB in ultrafast fiber lasers was summarized. The characteristics of the PDB in different dispersion regimes were analyzed. In addition, the features of the PDB of vector solitons, and that of multiple pulses were discussed.