Abstract: To solve low bale density of small square baler and heavy model with not flexible operation of large rectangular baler，middle rectangular baler was developed.Bale density of middle rectangular baler could reach up to 150 kg/m3，and it could be comparable with large square bundle but much lower energy consumption.Knotter system was key part of middle rectangular baler.Its movement relation and phase relation were complex.In the process of knot tying，movement phase relation and phase relation of each part must be highly coordinated.Strength of knot axis must meet the needs of knot.