A knot tying device, comprises: a support with a gripper element with one end of the gripper element rotatably connected to the support, and the other end capable of being pressed on and released from the support; a gripper mechanism to grip a free end of a strand of material and carry the strand of material to wrap around the support capable of rising and falling; and a stripping mechanism to separate the strand of material wrapped around the support from the support to form a knot. After the gripper mechanism carries the strand of material to wrap around the support at a position below the gripper element by at least half of a circle, the support falls so that the strand of material passes through the gripper element, and the gripper element is pressed on the support to make the free end of the strand of material fixedly gripped between the gripper element and the support.