In order to explore a suitable application of potassium chloride on jackfruit seedlings, this experiment used ‘Malaysia No.1’ grafted seedlings as the test materials to compare the effects of different potassium application on the growth, photosynthesis and nutrition accumulation of jackfruit seedlings. The results showed that with the increase of potassium level, the net photosynthetic rate of leaves, root parameters, biomass and nitrogen, phosphorus and potassium accumulation of organs generally increased first and then decreased, and the ratio of root/shoot increased. Compared with the K0 level, K1 level had the better growth effect which increased plant height, stem diameter, dry weight of leaf, stem, root and total plant, ratio of root/shoot by 25.41%, 16.77%, 13.73%, 56.64%, 230.24%, 60.15% and 157.89%, respectively. The growth of jackfruit was inhibited to varying degrees in excessive potassium application. K1 and K2 treatments also had higher values of net photosynthetic rate compared with the K0 level, which increased by 25.29% and 35.00%, respectively. With the increased application of potassium, photosynthesis was obviously inhibited and stomatal restriction was the main factor limiting photosynthetic efficiency of leaves at K4 level. Compared with the K0 level, K1 level had a significant effect on root growth which increased root length, root surface area, root diameter, root volume and root tip number by 10.65%, 84.34%, 71.92%, 215.25% and 46.84%, respectively. The excessive potassium application inhibited the root system in different degrees. The accumulation of nitrogen, phosphorus, and potassium in roots and leaves increased first and then decreased with the increase of potassium application. The change trend of accumulation of potassium in stems was similar. K1 treatment had a higher accumulation of potassium in organs compared with the K0 level, which increased by 203.84%, 100.77% and 64.32%, respectively. The root indexes were significantly positively correlated with dry weight and potassium accumulation of root except for the total root length. The net photosynthetic rate was significantly positively correlated with root diameter, root volume, root tip number, root dry weight and potassium accumulation of root. Stem and leaf potassium accumulation were significantly positively correlated with dry weight of stem and leaf, and root diameter. In summary, Root architecture is closely related to the uptake and accumulation of potassium in roots, and plays an important role in accumulation of root biomass and leaf photosynthesis. Suitable concentration of potassium fertilizer can improve crop nutrient uptake and biomass accumulation by improving root parameters and leaf photosynthesis. The proper potassium nutrition was 1 g/kg in this experiment which could be used as a reference for jackfruit seedlings fertilization.