The photo-induced charge transfer followed by the bidirectional phase transition in Fe-Co Prussian blue compounds is theoretically discussed. The result of ab initio calculations indicates that both the forward and backward transitions are triggered by photo-induced charge transfer excitations between Co-dgamma and Fe-depsilon orbitals, although different kinds of Co sites are excited. It is concluded that the charge transfer proceeds by way of the spsigma-ppi transition on the nitrogen atom, which can explain such a long range charge transfer between Co and Fe separated by about 5Angstrom.