Abstract: 【Objective】 This study aimed to reveal effects of altitude differences on metabolite distribution and taste quality of ancient tree tea， and discuss the synergistic effects of amino acids and catechins to establish quality grading indicators for ancient tree tea in small production areas， so as to provide a scientific basis for quality grading and origin certification of ancient tree tea.【Method】 A total of 125 samples of century-old ancient tree tea from five villages （Dijie， Bawai， Nanpo， Laozhai， and Nuowu） in the core production area （25 samples for each village） of Mengku， Yunnan were used as experimental materials. Components of 35 free amino acids were analyzed using an automatic amino acid analyzer， and 10 key amino acids with prominent variation and taste contribution among samples were selected for statistical analysis. Contents four types of catechins were determined by high performance liquid chromatography （HPLC）， and electronic tongue analysis was used to obtain taste response values of umami and astringency. The Wilcoxon rank-sum test， principal component analysis （PCA）， and support vector machine （SVM） were used to analyze the association between metabolites and tastes to establish the comprehensive quality index （KX index）.【Result】 The metabolites of Mengku ancient tree tea exhibited significant differentiation along the altitude gradients. In samples of high-altitude （≥1800 m） and low-altitude， the median of epigallocatechin gallate （EGCG） content was 4.43 mg/g， which was higher than that in samples of lower-altitude （4.27 mg/g）， while the median of theanine （The） content was extremely significantly reduced （P<0.01）. Contents of umami and sulfur-containing amino acids such as glycine （Gly） and methionine （Met） were generally enriched in the samples of low-altitude， whereas samples of high-altitude showed a slight increase in content of sulfur-containing and aromatic amino acids such as cysteine （Cys） and tryptophan （Trp）. PCA analysis showed that the first two principal components cumulatively explained 76.57% of the variance， with samples of high-altitude aligning with the loading direction of ester-type catechins such as EGCG， and samples of low-altitude associated with enrichment of The and Gly， indicating that altitude drove an overall rearrangement of the combination of ester-type catechins-umami amino acid. The SVM model revealed that when the Gly/EGCG ratio was above 0.45×10^-3， the umami response value （AAE） was relatively high； the average value in high-altitude samples （0.38×10^-3） deviated from the high-umami range， while the average Gly/EGCG ratio in samples of low-altitude （0.51×10^-3） was closer to the range. The KX quality index based on ratios of metabolites KX=0.6×（Gly/EGCG）+0.4×（The/Cys）， showed significant differences （P<0.05） between high altitude （172.23） and low altitude （229.71）， exhibiting a high consistency with sensory grading and electronic tongue evaluations； samples with higher KX values generally exhibited higher AAE values， moderate astringency values （AE1）， and better overall taste scores.【Conclusion】 Altitude significantly shapes the taste characteristics of Mengku ancient tree tea by influencing the accumulation of metabolites such as EGCG， The， Gly， and Cys as well as their ratio relationships. Characterized by the accumulation of ester-type catechins such as EGCG， the tea of high-altitude shows stronger astringency； while tea of low-altitude， characterized by the enrichment of umami amino acids such as The and Gly， resulting in a more pronounced umami taste. The KX index can serve as an auxiliary tool for quality grading and origin certification of Mengku ancient tree tea at small-scale production areas.