With the weak-coupling approach in molecular vibrational normal coordinate, the charge transmission under external optical field through molecular nano-junction is studied. From the charging energy point of view, the sequential charge transmission is analyzed and the current-voltage characteristics under external optical field are studied. It is found that the vibrational effect of molecule is the key factor of the removal of Franck-Condon blockage under external optical field. Furthermore, the effects of charging energy and intra-molecular vibrational energy redistribution (IVR) in charge transmission under external optical field are studied in detail.