The photoinduced processes of phytochromes have received great research interest for their important biological functions. Phytochromobilin (P Phi B), one of the most important phytochrome chromophores, was selected as the prototype to study its photoinduced isomerization. The nonadiabatic dynamics of P Phi B from the Pr configuration in the gas phase was investigated by the surface hopping method at the OM2/MRCI level. In the excited state, isolated P Phi B does not display the rotation of the two terminal five-membered rings (ring A and ring D), which is assumed to govern the Pr -> Pfr process in the protein. Instead, two S-1/S-0 conical intersection seams (CI01 alpha and CI01 beta) characterized by the rotation of the two central rings (ring B and ring C) were proven to play essential roles for the photoisomerization of P Phi B in the gas phase. These two conical intersections (CI01 alpha and CI01 beta) are accessible by the twisting motions of the C9-C10 and C10-C11 bonds, respectively. The CI01 alpha, and CI01 beta seams, instead of their minimum-energy points, are responsible for the nonadiabatic dynamics. For both channels, the trajectories may propagate forward to the isomerization products or backward to the original Pr configuration after the Si -> So hops.