A fixation device for an intramedullary nail for proximal femoral fractures, including a main intramedullary nail, a combined locking nail, and cortical bone screws. The main intramedullary nail has a longitudinal axis, a proximal end, and a distal end with a tip. The proximal end includes a through hole for assembling the combined locking nail. The distal end includes an oblong through hole and a circular through hole, both for receiving the cortical bone screws. The combined locking nail includes a head, a distal end of the locking nail, a sleeve, a connecting block, and a cover cap. The connecting block includes a left-handed external thread. The cover cap includes a left-handed internal thread matching with the external thread of the connecting block, a symmetrical sliding plane, and a protrusion block disposed at the end of the symmetrical sliding plane.