We perform ab initio calculations on two different transition mechanisms of the bcc-to-hcp phase transition in Fe under pressure distinguished by the occurrence of the metastable fcc intermediate phase on the transition path, that is, the bcc-hcp and the bcc-fcc-hcp. The calculated results indicate that the occurrence of the fcc intermediate state during the transition is energetically unfavorable, which is consistent with the recent in situ XRD experiments. The enthalpy barrier of the fcc-hcp increases with pressure increasing, which indicates that the pressure tends to impede the transformation from fcc to hcp phase in Fe. The details of the structural and magnetic behaviors of the intermediate states during the transition are investigated, which indicates that there are complex magnetism transitions during the phase transition. The physical origins of the influence of magnetism on the phase transition are discussed. Moreover, the origin of the occurrence and evolution of the fcc metastable structure during the transition in the MD simulations are also discussed.