Abstract: Six cultivars of Dahlia pinnata Cav. were used to investigate the effects of high-temperature stress (40℃/35℃, 14 h/10 h (day/night), 2 d) on morphological and physiological indices. Correlation analysis, principal component analysis, and membership function analysis were applied to calculate the comprehensive evaluation value D for heat resistance and stepwise regression analysis was used to screen indicators of heat tolerance. Results showed: (1) After high-temperature stress, the F_v / F_m value and chlorophyll content decreased in leaves. Ascorbic acid and reduced glutathione contents increased in leaves and decreased in stems, while soluble sugar and soluble protein contents decreased in both leaves and stems. Total phenol content increased significantly in leaves and decreased significantly in stems, while anthocyanin content increased in leaves and stems. (2) According to the comprehensive evaluation value D for heat resistance, among the six varieties, ‘SLGZ’ showed the strongest heat resistance, ‘KEW’, ‘QKL’, and ‘YH-C11 showed medium heat resistance, and ‘AGR-C1’ and ‘YY-C5’ showed the weakest heat resistance. (3) Regression analysis indicated that semi-lethal temperature and chlorophyll content of leaves, soluble protein and ascorbic acid content of stems, and the heat damage index could be used as indicators of heat tolerance in dahlias.