The present disclosure provides an apparatus for locking a vehicle seat-rail that improves the assembly convenience of a lever assembly and enhances space utilization of the seat-rail upper portion, and an assembling method. The apparatus includes a fixing bracket that is fixed to an outer surface of a moving rail; and an unlock lever that is rotatably coupled to the fixing bracket at an interruption. In particular, a rotation stopper is formed to protrude from a side surface of the unlock lever, and a fixing stopper is provided in a rotation path of the rotation stopper to regulate the excessive rotation of the unlock lever in one direction at the time of rotation of the unlock lever.