A wheel cylinder adjuster includes plural actuating discs able to match different-sized wheel cylinders, and a driving wrench able to be jointed with the actuating disc. The driving wrench has two ends of its shank respectively provided with right-handed threads and left-handed threads on the surface, having a crosswise insert hole respectively bored at an outer side of the right-handed threads and of the left-handed threads, and the shank has two ends respectively protruding outward to form a positioning rod to be inserted in the insert hole of the actuating disc. Two positioning sleeves are respectively combined with the right-handed threads and the left-handed threads at the two ends of the shank. A holding rod is inserted in either one of the two insert holes at two ends of the driving wrench, and a positioning stop plate is provided to fix one of the two positioning sleeves.