The invention relates to a reducing control mechanism of a loom, comprising a base, wherein the base is provided with a plurality of linear guide rail assemblies, the linear guide rail assemblies are uniformly distributed by taking an opening at the center of the base as the center of a circle; the end part of each linear guide rail assembly is provided with a size ring; the size rings penetrate through the opening at the center of the base to be combined into a closed region below the base; a disk grooved cam provided with the opening at the center is erected on each linear guide rail assembly; the disk grooved cams are respectively provided with line grooves which are in one-to-one correspondence with the linear guide rail assemblies; pin shafts of the linear guide rail assemblies are arranged in the line grooves and can slide along the line grooves; incomplete gears are arranged on the circumference of each disk grooved cam; rotation centers of the incomplete gears are coincided with the rotation centers of the disk grooved cams, and the incomplete gears are driven to rotate along axis of the disk grooved cams by virtue of a motor. By adopting the reducing control mechanism of the loom disclosed by the invention, dynamic control on the calibre of a fabric on a circular weaving machine can be realized.