Abstract: In order to study the effects of top pruning time and retained stem height on the quality of Pinus koraiensis seedlings, a top pruning test was carried out with 4-year bare root seedlings of Pinus koraiensis as materials, and 3 retained stem heights（20, 25, 30 cm） and 5 top pruning times（April, May, June, July, August） were set. The root collar diameter, biomass, shoot to root ratio, root length, root surface area and root volume of seedlings under different top pruning treatments were measured. The results showed that top pruning treatment in the middle of the growing season（June） could significantly increase the root collar diameter growth of Pinus koraiensis seedlings by 23. 68% compared with the control, but the effect of top pruning treatment at other times was not obvious. The top pruning treatment had no effect on the increase of biomass, the decrease of shoot to root ratio and the growth of thick roots, but had a significant effect on the growth of fine roots of Pinus koraiensis seedlings. The length of fine roots, surface area of fine roots and volume of fine roots were increased by 18. 27%-84. 95%, 9. 38%-75. 01% and 1. 28%-64. 68%, respectively, compared with the control. The top pruning in the middle of the growing season（June） increased the root collar diameter growth of Pinus koraiensis seedlings, promoted the growth of fine roots and promoted the quality of Pinus koraiensis seedlings to a certain extent.