A pulley includes a tensioning device and a ratchet device disposed in the tensioning device. The tensioning device includes a first shell, a second shell installed to the first shell, and a first compartment defined in a side thereof. The tensioning device further includes a first block and a second block located in the first compartment, with the first and second blocks adapted to pivot in the tensioning device. The ratchet device includes an actuating element, a ratchet, a first gripping element and a second gripping element, with the ratchet, the first and second gripping elements mounted onto the first block in sequence, with the actuating element provided to control the pivoting direction of the ratchet as to limit the direction of pulling a rope, which is winded between the first and second gripping elements.