Abstract: Absrtact: K plays an important role in fruit growth and development. It is of great theoretical and practical significance to clarify the regulation mechanism of Kon acid metabolism in apple fruit. In this study, the field positioning experiment of 2 years was used, and the 7-year-old 'Vallee spur Del.' apple tree was used as the research object. The K-free fertilizer was used as the control (CK), and 4 treatments were set according to the K2O dosage, namely T1 (0.20 kg/ plant), T2 (0.35 kg/plant), T3 (0.50 kg/plant) and T4 (0.65kg/plant). All of the treatments were applied three times, namely including initial fruiting stage (30%), fruit expansion stage (40%) and fruit ripening stage ( 30%). Four radial 30～40 cm deep ditches were excavated in the tree pan during fertilization and irrigated after fertilization. The regulation mechanism of K on acid metabolism of 'Red Delicious' apple fruit was discussed through the determination and analysis of K accumulation, fruit quality, endogenous hormone content, organic acid content and acid metabolism related enzyme activities in different organs during its growth and development. The results showed that fruit weight, Vc content, soluble solids content and hardness increased with the increase of K application, but there was no significant difference between T4 and T3 (P < 0.05). Compared with CK, the single fruit weight, Vc content and hardness of T4 increased by 24.40%, 14.60% and 7.68% in 2016, and also increased by 16.30%, 14.55% and 6.93% in 2017, respectively. The titratable acid content decreased with the increase of K application, and the difference of T4 was the most significant (P < 0.05). Compared with CK, the titratable acid content of T4 in 2016 and 2017 decreased by 26.47% and 18.18%, respectively. At different growth stages, the order of K accumulation was T4 > T3 > T2 > T1 > CK, and there was no significant difference between T4 and T3 (P < 0.05). The absorption and storage of K+ in leaves are mainly concentrated in the early growth stage. With the growth and development of trees, K accumulated in leaves gradually transfers to fruit branches and other parts for their growth and development, resulting in the gradual decrease of K+ content in leaves. K application increased the contents of ZR, IAA and GA in fruits from 30 to 120 days after anthesis, and ABA content at 150 days after anthesis. T3 had the most significant effect on the content of ABA (P < 0.05). Compared with CK, ABA content of T3 increased by 15.28% and 18.08% in 2016 and 2017 at 150 days after anthesis, respectively. The content of malic acid and citric acid decreased with the increase of K application. Compared with CK, the content of malic acid and citric acid of T4 at 150 days after anthesis decreased by 34.68% and 12.3% in 2016, and also decreased by 32.60% and 16.0% in 2017, respectively. The activities of MDH and PEPC decreased with the increase of K application, while the activities of PEPCK and NAD-cy ME increased with the increase of K application from 120 d to 150 d after anthesis. The activities of PEPCK and NAD-cy ME from T4 at 150 days after anthesis increased by 13.3% and 16.9% in 2016, and also increased by 18.9% and 20.3% in 2017, respectively. At the ripening stage, single fruit weight and soluble solids content were positively correlated with IAA and ABA (P < 0.01), titratable acid content was positively correlated with GA, ZR and IAA (P < 0.05), and negatively correlated with ABA (P < 0.01). Malic acid and oxaloacetic acid were positively correlated with ZR and IAA (P < 0.01) and negatively correlated with ABA (P < 0.01). MDH and PEPC were positively correlated with IAA (P < 0.01) and negatively correlated with ABA (P < 0.05). PEPCK was positively correlated with ABA (P < 0.01). NAD-cy ME was negatively correlated with IAA (P < 0.05) and positively correlated with ABA (P < 0.05). In conclusion, K affects the activity of enzymes related to organic acid metabolism by regulating the content of endogenous hormones in fruits, and then affects the acid metabolism of fruits, reduces the organic acid content in fruits, and improves the fruit quality.