An exchangeable cylinder lock includes a plug shell connected to a pin chest. A locking pin is disposed in a radial pin hole in the pin chest and is biased to protrude from an aperture in a cover plate and to engage a lock housing. The locking pin is depressable to retract for disengaging from the lock housing. A locking ring is disposed around the plug proximate to the locking pin, and has at least one indentation. When the plug is rotated using a right key, the locking ring can be rotated to align the indentation with the locking pin so as to permit depression of the locking pin, or to move the indentation away from the locking pin so as to prevent depression of the locking pin.