The content and proportion of sugar and acid substances in tomato fruits directly affect its flavor quality. Previous studies have shown that appropriate concentration of exogenous 5aminolevulinic acid (ALA) can promote the ripening of fruits and improve their aroma quality. We conducted this experiment to explore the effects of different concentrations of exogenous ALA (0, 100, 200 mg·L^-1 ) on the development of tomato fruits and their sugar and acid qualities. Tomato fruits (Solanum lycopersicum cv. Yuanwei No. 1) grown in greenhouse were used as test material, ALA solution was sprayed on the surface of the fourth ear fruit at the fruit setting stage. 10 d after pollination and the fruit morphology, peel color, sugar and acid contents in different fruit tissues were determined. The results showed that: (1) the exogenous ALA solution significantly promoted the increase of tomato fruit transverse and longitudinal diameters and the increase of fruit weight per fruit; the exogenous ALA treatment also significantly reduced fruit hardness, promoted fruit softening and improved fruit taste; the exogenous ALA treatment also increased fruit V_C and soluble solids content. (2) The results of sugar fractions in different parts of fruit tissues (including flesh, columella and septum) showed that exogenous ALA treatment significantly increased the total soluble sugar content (including fructose, glucose and sucrose) and facilitated the accumulation of sugar into the fruit flesh. (3) Among the organic acids, except for the increase of tartaric acid content, exogenous ALA could reduce the contents of other acid components in the tissues of various parts of the fruit. Therefore, the ratio of sugar to acid could be significantly improved in the pulp parts of tomato fruits, and the fruit flavor quality was promoted. It was found that exogenous application of 200 mg·L^-1 ALA during the development process of tomato fruits can not only promote fruit development, fruit weight and coloration, but also improve the formation of fruit sugar and acid quality, which enhance the appearance quality.