The utility model provides a compaction mechanism and a seeder. The compaction mechanism comprises a rack and a compaction rotating shaft, the rack is rotationally connected with the compaction rotating shaft, the compaction mechanism further comprises a compaction shaft sleeve and two hollow circular-truncated-cone-shaped compaction rolling wheels, guide holes are formed in the compaction rollingwheels, an annular cylinder is arranged on the outer circumferential surface of the compaction shaft sleeve in a protruding mode, and the guide holes are connected with the annular cylinder in a sliding mode. The compacting mechanism further comprises an adjusting assembly. According to the compaction mechanism and the seeder, the adjusting assembly is arranged to provide control pre-tightening force for the first pressure spring and the second pressure spring through the adjusting knob, so that the axial position of the compaction roller is adjusted to adapt to ridge width adjustment withina certain range.