The subsoil compaction device of bowl seedling tray cave-hole for rice seedling nursery planter was developed for soft and hard seedling tray. The device control system based on AT89C51 microcontroller used a stepping motor and a transporting-tray limit switch to control seedling tray feeding, and used a restrict limit switch and an align proximity switch to conduct seedling tray alignment and interaction between subsoil in the seedling tray cave-hole and the roller finger. With these steps, subsoil compaction was realized on this device. Through various testing on the compaction device, the optimum working parameters of the compaction device were obtained. The seedling tray alignment test showed that the system realized exact match between the seedling tray cave-hole and roller finger during the subsoil compaction operation at a productivity of 500 trays per hour and an advance angle arc length of 1mm, and the alignment rate of 98%, which met the technical requirements for subsoil compaction of bowl seedling tray cave-hole. The seedling nursery test showed that subsoil compaction of bowl seedling tray cave-hole improved the quality of seedlings and ensured the integrity and tightness of bowl seedling root-soil complex, which was most helpful for planting operations when the depth of subsoil compaction was 6mm.