The invention discloses an automatic contracting device for a rectangular sealing ring of an automobile brake caliper. The automatic contracting device comprises a tool base, a sealing ring grabbing mechanism and a semicircular sealing ring contracting mechanism; the tool base is provided with a positioning hole used for positioning the rectangular sealing ring; the sealing ring grabbing mechanism comprises a grabbing arm which can vertically telescope, and an annular groove used for allowing the rectangular sealing ring to be clamped in is formed in the bottom of the grabbing arm; the semicircular sealing ring contracting mechanism comprises two semicircular contracting blocks and a driver used for driving the two semicircular contracting blocks to do left-right relative motion, and each semicircular contracting block is provided with a semicircular groove acting on the outer circle of the rectangular sealing ring. According to the device, the rectangular sealing ring evenly contracts according to the complete circle shape in the production process, and therefore damage such as serious deformation of the rectangular sealing ring can not be caused.