The invention belongs to the field of food processing mechanical equipment, and particularly relates to a kelp knot tying machine applied to kelp processing operation and a knot tying method. The kelp knot tying machine comprises a rack, a PLC control system and an air actuated control system, wherein a feeding device, a material conveying device, a knot tying device and a cutting device are arranged on the rack. According to the kelp knot tying machine, the disc type feeding device driven by a step motor and three pairs of knot tying fingers, which form a unique cutter structure, are closely matched to simulate the manual knot tying process to complete the kelp knot tying and quick and convenient cutting, so that the knot tying efficiency, the success and the consistency are effectively improved. Meanwhile, the design of the parts abides by the principles of serialization, universalization and standardization, so that the manufacturing, the mounting and the maintenance of the equipment become simple and quick.