The invention discloses an adjustable sizing block, comprising a steel wire check ring, a locking block, a locking bolt, a base, spanner holes, a nut sleeve and a groove face top block. The steel wire check ring is arranged in the groove face top block, and the nut sleeve is arranged on the base. Four spanner holes are uniformly distributed along the circumference of the nut sleeve (only one is drawn in the figure) so that the nut sleeve can be rotated by a matched spanner to ascend or descend a slotted surface press block. When the nut sleeve is rotated to an ideal position, the locking boltis rotated by using the other end of the matched spanner to tightly press the locking block on the thread part of the sleeve, wherein the inner face of the locking block is provided with screw threads with the same specification as that of the sleeve, thereby the slotted surface press block is prevented from gliding downwards due to workpiece vibration or other conditions. In order to fix the base with a boring mill worktable, the two sides of the base are provided with elongated slots for installing T-shaped bolts.