A vehicle seat rail locking device restricts movement of an upper rail along a longitudinal direction of a lower rail fixed to the body of the vehicle. The locking device includes lock units moving together with the upper rail, disposed movably in a vertical direction between the lower and upper rails, and for performing a locking operation, and a lock guide moving together with the lock units and for guiding vertical movement of the lock units. Each lock unit is independently inserted into one of a plurality of lock holes formed in the lower rail to fix the upper rail onto the lower rail. A user's safety may be improved by independently forming a plurality of lock members fitted into lock holes of a lower rail and fixing one of lock members into the lock hole to perform a locking operation even when a seat rail position deviation occurs.