The invention relates to a theta knot tying device and method of straw. The theta knot tying device is characterized by comprising a rack, a knot tying nozzle, a line clamping wheel, a slider crank-link mechanism, a knot clamp winding mechanism, a cutter crank-link mechanism, a cable apparatus mechanism, knot clamps connected in a string and a bundling rope arranged outside the rack. The knot tying nozzle is connected to the rack in a rotating mode, and the inlet of the knot tying nozzle is provided with a bundling rope groove and a knot clamp groove. The line clamping wheel is connected to the rack in a rotating mode, and a rope pressing apparatus corresponding to the line clamping wheel is arranged. The slider crank-link mechanism comprises a crank connected to the rack in a rotating mode, and the crank is connected with a slider through a link. The knot clamp winding mechanism comprises a rotating winding shaft, and the string of knot clamps are wound around the winding shaft. A support is connected to the front end of the slider, and a conveying wheel is connected to the support. The cutter crank-link mechanism comprises a cutter crank connected to the support, and the cutter crank is connected with a cutter bar in a rotating mode through a cutter link. Because the planar link mechanisms are adopted for controlling the components to move, the theta knot tying device is simple in structure and stable in working, and guarantees that knots are easier to tie and not prone to loosening.