Root-cutting grafting is a new method in cucurbitaceous seedlings grafting. Compared with the traditional methods, root-cutting grafting seedling grows with stronger root, resulting in a greater production in a longer harvesting period. Recently many grafting robots can work well in grafting processing. But the lack of planting grafted seedlings automatically has become a problem in the automation of industrialized nursery process. To solve this problem, a mechanism of automatically planting root-cut grafted seedlings of cucurbitaceous was developed, which included soil hole-making unit, seedling-moving unit, seedling-planting unit and other parts. It can be added to the current grafting robot, and automatically planting seedlings grafted by the robot. The soil hole-making unit can hit a hole at the center of each cell of the seedling tray filled with soil. And the seedling-moving unit can transport the grafted seedlings from the grafting area to the planting area. The seedling-planting unit was the key part of the mechanism, which can plant the seedlings automatically. Some simulations were done with the software of Solidworks. And the simulation results showed that the velocity ratio of prismatic cylinder in the seedling-planting unit to the joint cylinder in the seedling-planting unit should be between 0.22 and 0.55. Experiments were conducted in laboratory, and the results showed that the success rate of planting seedlings was up to 92%, and the longest time of one planting was 7s. Experiment in laboratory showed that the designed planting device can work well with the current grafting robot in an automatic and efficient way.