The invention relates to a lock cylinder measuring tool for an antitheft door. The lock cylinder measuring tool comprises the components of a main ruler (1), a left auxiliary ruler (2), a right auxiliary ruler (5), a left spacing assembly (3), a right spacing assembly (6) and an origin positioning assembly (4). The lock cylinder measuring tool belongs to the technical field of measurement control. The lock cylinder measuring tool can accurately measure the inner length and the outer length of the lock cylinder of the antitheft door on condition that the lock cylinder of the antitheft door is not detached, so that a locksmith does not require to detach and mount the lock cylinder of the antitheft door before ordering, thereby greatly simplifying a business service for replacing and upgrading the lock cylinder by the locksmith, and improving operation efficiency. The lock cylinder measuring tool performs positioning on the lock cylinder by means of a tool tightening groove of a cross recessed countersunk head screw, thereby determining a measurement origin, realizing accurate positioning and simple operation, and satisfying a business requirement. The lock cylinder measuring tool has a structure which comprises the main ruler and the auxiliary ruler and is similar with a vernier caliper. The lock cylinder measuring tool can acquire multiple length data which are not in a same plane with the measurement origin in one time, and furthermore can be used for length measurement in similar occasions.