The invention relates to a seat-adjustable ISOFIX (International Standards Organisation FIX) system of vehicle-mounted children safety-restraint system. The seat-adjustable ISOFIX system comprises a base frame, a FIX connector, an adjusting stirring plate and an adjusting mechanism. The adjusting stirring plate is connected with a FIX connector supported on the bottom of the base frame in a sliding way through an adjusting mechanism; the adjusting mechanism comprises a front support plate, a rear sliding plate, a gear shifting slide gear, a pneumatic spring, a gear locking post and a gear locking plate, wherein the front support plate is fixedly connected with the front end of the base frame, the gear locking plate is arranged on the front support plate, one end of the gear shifting slide gear is supported in the shift locking plate and is in a state of sliding along a front-to-back direction of the base frame, the other end of the gear shifting slide gear is connected with the FIX connector via the rear sliding plate, the front support plate and the rear sliding plate are connected by the pneumatic spring, the lower end of the adjusting stirring plate is connected to the front support plate via the spring, and the adjustment stirring plate is provided with the gear locking post corresponding to the notch of the shifting slide gear. The seat is adjustably mounted and fixed, and the seat is simple and compact in structure, less in transmission components, steadily locked and time-saving and labor-saving in operation.