A form assembly of a form ejection vehicle for quick construction of long flange plates of railway simply-supported T-beams is provided to further improve construction efficiency of the long flange plates of the simply-supported T-beams and to ensure construction safety. The form assembly comprises a bottom form, a vertical frame, a connecting frame, a lifting form and a profiling form; the vertical frame is located on the transverse outer side of the bottom form and is rigidly connected with the bottom form; the connecting frame is detachably mounted at the top end of the vertical frame; the profiling form is detachably mounted on the inner side of the vertical frame; the lifting form is detachably hung to the inner lower portion of the connecting frame.