The invention provides a loss reduction laying and compacting device of a potato and onion harvester. The loss reduction laying and compacting device includes a machine frame, a floating device and asoil pressing walking device. The frame machine includes a stand column, a reinforcing rod, a vertical plate, a cross rod, a connecting rod and a lug. The loss reduction laying and compacting device is characterized in that the floating device includes a supporting arm, a floating arm, an adjusting screw sleeve, an adjusting lead screw and a compression spring, the two U-shaped sides of the supporting arm of a U-shaped structure extend toward a same direction perpendicular to a U-shaped plane, lugs III are arranged at the extended U-shaped sides of the supporting arm, and circular holes with equal diameter and coaxial axis are formed in the lugs III. The soil pressing walking device includes a soil pressing roller, right-angle frames, a soil scraping plate and a ground wheel. The soil pressing roller is supported in a round hole in the end, arm away from a lug V, of the floating arm through pins at the two ends, and the two right-angle frames with right-angle cross-sections are installed on the lower surface of the end, equipped with the soil pressing roller, of the floating arm through a right-angle side plate. The effects of reducing damage and preventing damage are good, the dead weight of the machine can be used to compact the ground surface, the covering rate of potatoes and onions is reduced, and the harvest quality is improved.