A fast-positioning brake slave cylinder adjuster includes a driving rod, an operation grip, a positioning mechanism, and a baffle. Therein, the driving rod engages a driving disk for being coupled with a brake slave cylinder, while the driving rod has a first thread section and a second thread section with different rotation directions. The operation grip drives the driving rod. The positioning mechanism is mounted around the driving rod, and capable of generating a clutching movement. When being de-clutched with the driving rod, the positioning mechanism is allowed to fast move axially along the driving rod; when being clutched with the driving rod, the positioning mechanism is allowed to be screwed to the first or second thread sections. The baffle is mounted around the outer periphery of the positioning mechanism. Therefore, with a fast adjusting function achieved, the adjusting operation is facilitated.