Straw returning and chopping device and operation method

The straw returning and chopping device comprises a rack, a cam, a knife rest, a weed pulling frame, a first connecting rod, a second connecting rod, a sliding block and a swing sleeve, the two sides of the rack are provided with two symmetrical side guide grooves, the front sides of the side guide grooves are provided with a weed chopping cavity, the end face of the upper portion of the weed chopping cavity is provided with four guide holes which are symmetrical front, back, left and right, and the outer side of each guide hole is provided with a vertical rod; a motor plate is arranged on the upper portion of the left inner vertical plate, a sliding groove is formed in the inner side of the left inner vertical plate, a first inner vertical plate hole is formed in the upper portion of each inner vertical plate, a second inner vertical plate hole is formed in the lower portion of each inner vertical plate, and an outer vertical plate is arranged on the rear side of each transverse rod. A first outer vertical p