A strap wrench is used to drive a cylindrical object and contains: a handle including two recesses and a longitudinal slot; a strap including a first end portion and a second end portion which are inserted into the two recesses; a guiding rod longitudinally inserted into the handle; a supporting block rotatably connected with a first segment and located inside the strap; a movable member disposed on the guiding rod and moving between a first position and a second position, the movable member joining with the strap; a push switch slidably fixed in the longitudinal slot and rotatably connected with the movable member; and a spring for pushing the movable member to move toward the second position. Thereby, the movable member is driven by the engaging block to move between the first position and the second position so as to operate the strap wrench easily.