A spanner capable of controlling the extension lengths of a through bolt mainly solves the problem of the prior art that the through bolt is installed manually, and therefore the extension lengths of the two ends of the through bolt are different. The spanner comprises a spanner body, a nut clamping sleeve and a positioning mechanism, wherein the nut clamping sleeve is embedded in the spanner body, a second stepped hole used for clamping a nut is formed in the center of the nut clamping sleeve, the positioning mechanism is arranged on the end face of the side, provided with steps, of the head of the spanner body and comprises a positioning plate, a fastening screw and a positioning sliding block, a slotted hole is formed in the middle of the positioning plate, a scale plate is arranged on one side of the positioning plate, the zero graduation line of the scale plate is flush with the end face of the non-stepped side of the head of the spanner body, the fastening screw and the positioning sliding block are located on the two sides of the slotted hole and are in threaded connection, and sliding and locking of the positioning sliding block can be achieved by rotating the fastening screw. By the adoption of the spanner, the extension lengths of the two ends of the through bolt can be controlled accurately during installation.