A seat rail apparatus for a vehicle may include a guide rail fixedly installed along a lower rail mounted to a bottom surface of the vehicle, a lock assembly installed to be movable along the guide rail in an unlocked state and selectively locked and unlocked with and from the guide rail as a locking pin is moved forwards and rearwards, a driving assembly integrally installed at an upper rail mounted to a cushion frame, wherein the driving assembly locks and unlocks the lock assembly with and from the guide rail by operating the locking pin and locks and unlocks the lock assembly such that the lock assembly is operated integrally with the lock assembly according to a walk-in lever, and a stopper provided at the upper rail, for contacting the lock assembly locked with the guide rail when the seat returns after a walk-in operation to stop the seat.