The utility model discloses an automatic constriction device of automotive brake caliper rectangular seal ring, pick mechanism and sealing washer semicircle shrink mechanism including frock base, sealing washer, be equipped with the locating hole that advances line location to rectangular seal ring on the frock base, the sealing washer snatchs the mechanism including the arm that snatchs that canstretch out and draw back from top to bottom, the bottom of snatching the arm is equipped with the annular that makes the rectangular seal ring card go into, relative motion's driver about sealing washer semicircle shrink mechanism includes two semicircle shrink pieces and drives two semicircle shrink pieces, semicircle shrink piece is equipped with the half slot with the effect of rectangular seal ring excircle. The utility model discloses make in process of production, rectangular seal ring evenly contracts according to whole circular form, can not lead to damages such as rectangular seal ring severe deformation.