Abstract: Taking Hydrangea macrophylla Seringe as the test material, the effect of K fertilizer on the cutting seedling growth was studied by the root application method. The results showed that it did not exist significantly difference in the root rate and the length of root between the short branch and the lon branch treated by K fertilizer. However, it existed significantly difference in the number of the root of the cutting seedling cultivated by the short branch after the treatment of K fertilizer. Compared to control (CK), the number of root of the cutting seedling treated by 0.125, 0.25 and 0.3125 g/kg decreased significantly 81.46%, 72.35% and 68.10%, respectively. The study also showed that the weight of the cutting cultivated by the long branch was significantly higher than the cutting seedling cultivated by the short branch. The results from this study would be helpful to the reproduction of Hydrangea macrophylla Seringe.