Abstract: This study determined the mechanism of the effect of exogenous GA₃ on the development of fleshy carrot roots and to provide theoretical base for efficient and safe cultivation of summer carrots planted on plateau areas of the Northwest China. To do so, the study analyzed the effect of exogenous GA₃ on immature bolting carrot cultivar ('Xinxiusanhong') in terms of fleshy root thickening, lignification and endogenous hormones contents. The results showed that weight, growth rate and sink activity of fleshy roots of carrot significantly reduced after spraying with 5.78 × 10 4 mol·L^-1 of exogenous GA₃ once during vigorous leaf growth period (45 days after seedling emergency) or during root thickening period (75 days after seedling emergency). The weight, growth rate and sink activity of fleshy roots of carrots also significantly reduced following 3 rounds of continuous spray of 5.78 × 10^-4 mol·L^-1 of exogenous GA₃ at seedling stage (25 days after seedling emergency), vigorous leaf growth period and root thickening period. However, the level of lignification of fleshy roots of carrot was enhanced by spraying 5.78 × 10^-4 mol·L^-1 of exogenous GA₃. Compared with CK (the control), fleshy carrot root contents of endogenous GA₃, IAA, GA₄, ABA, DHZR, IPA were significantly reduced and ZR significantly increased at seedling stage after spraying with exogenous GA₃. GA₃, IAA, GA₄ and IPA contents significantly decreased at vigorous leaf growth stage while endogenous IAA, GA₄, ZR contents significantly increased during the period of rapid thickening of fleshy carrot roots. Also, ZR content increased at vigorous leaf growth stage of carrot. The ratios of endogenous IAA/ZR and GA₄/ZR significantly decreased before the period of rapid thickening of fleshy carrot roots after spraying one to three times with exogenous GA₃. IAA/ZR and GA₄/ZR ratios significantly increased during the period of rapid thickening of fleshy carrot roots following several rounds of spraying with exogenous GA3. Exogenous GA₃ influenced the development of fleshy roots of summer carrots cultivated in the plateau region of the Northwest China by changing the content and balance of endogenous hormones.