A seat rail locking device for a vehicle, the seat rail locking device comprising: a fixed rail adapted to be fixedly mounted to a floor panel; a movable rail fastened to the fixed rail to be movable in forward and backward directions; a support plate mounted on the movable rail for assembling with a seat cushion; a lock pin guide provided in a hollow structure in which a spring backing part is formed, and inserted into and fastened to the movable rail, wherein the lock pin guide includes: a hollow guide pipe in which the spring backing part is formed; and a mounting plate integrally formed on a circumferential portion of an upper end of the guide pipe, and seated on and fastened to an outer circumferential surface of an assembling hole when a lower portion of the guide pipe is inserted into and fastened to the assembling hole formed in the movable rail, wherein a bottom surface of the mounting plate is formed with a release prevention hook, and the outer circumferential surface of the assembling hole of the movable rail is formed with a hook fastening hole into which the release prevention hook is inserted and fastened; a lock pin provided in a structure in which a spring support part is formed to protrude from an outer diameter portion, and inserted into the lock pin guide to be movable in upward and downward directions; a spring disposed between the spring support part and the spring backing part of the lock pin guide to be compressible; a track lever fastened to the support plate by a hinge and connected to an upper end of the lock pin; and a locking plate fastened the lower end of the lock pin, moving in an upward direction when the lock pin moves in the upward direction to be inserted into and locked to a locking groove having an opened lower portion formed at an inner end of the fixed rail, and moving in a downward direction together when the lock pin moves in the downward direction to be separated and unlocked from the locking groove of the fixed rail. a fixed rail adapted to be fixedly mounted to a floor panel; a movable rail fastened to the fixed rail to be movable in forward and backward directions; a support plate mounted on the movable rail for assembling with a seat cushion; a lock pin guide provided in a hollow structure in which a spring backing part is formed, and inserted into and fastened to the movable rail, wherein the lock pin guide includes: a hollow guide pipe in which the spring backing part is formed; and a mounting plate integrally formed on a circumferential portion of an upper end of the guide pipe, and seated on and fastened to an outer circumferential surface of an assembling hole when a lower portion of the guide pipe is inserted into and fastened to the assembling hole formed in the movable rail, wherein a bottom surface of the mounting plate is formed with a release prevention hook, and the outer circumferential surface of the assembling hole of the movable rail is formed with a hook fastening hole into which the release prevention hook is inserted and fastened; a hollow guide pipe in which the spring backing part is formed; and a mounting plate integrally formed on a circumferential portion of an upper end of the guide pipe, and seated on and fastened to an outer circumferential surface of an assembling hole when a lower portion of the guide pipe is inserted into and fastened to the assembling hole formed in the movable rail, wherein a bottom surface of the mounting plate is formed with a release prevention hook, and the outer circumferential surface of the assembling hole of the movable rail is formed with a hook fastening hole into which the release prevention hook is inserted and fastened; a lock pin provided in a structure in which a spring support part is formed to protrude from an outer diameter portion, and inserted into the lock pin guide to be movable in upward and downward directions; a spring disposed between the spring support part and the spring backing part of the lock pin guide to be compressible; a track lever fastened to the support plate by a hinge and connected to an upper end of the lock pin; and a locking plate fastened the lower end of the lock pin, moving in an upward direction when the lock pin moves in the upward direction to be inserted into and locked to a locking groove having an opened lower portion formed at an inner end of the fixed rail, and moving in a downward direction together when the lock pin moves in the downward direction to be separated and unlocked from the locking groove of the fixed rail.