Longan (Dimocarpus longan Lour.) is an important economic crop in tropical subtropical areas. Under normal circumstances, longan can become flowers only when induced by low temperature in winter together with appropriate measures to control the winter tips, but potassium chlorate, as the only known compound that can induce longan to become flowers in the opposite season, the anatomical process and physiological mechanism of the potassium chlorate-regulated flower formation are not clear, and the carbon and nitrogen nutrition is closely related to flower formation. Carbon and nitrogen nutrition are closely related to flower formation, therefore, analyzing the changes of carbon and nitrogen content of longan by potassium chlorate is also an important perspective to reveal the mechanism of flower formation. In this study, we used Shixia longan (Dimocarpus longan Shixia) as the test material, and induced the antiseasonal flower formation of longan by soil application of potassium chlorate to investigate the anatomical changes of the terminal buds of the antiseasonal longan during flower formation, and analyzed the changes in the C and N contents as well as the relationship between C/N and the development of the terminal buds. By observing the growth point of the terminal buds in the control group, which always showed a more pointed leaf bud state, the terminal buds after potassium chlorate treatment clarified the three stages of flower bud differentiation according to the changes in the morphology of the growth point: the stage of physiological differentiation of flower buds, the stage of inflorescence formation of flower buds, and the stage of floral organ formation. The carbon content of terminal buds treated with potassium chlorate showed a continuous increasing trend, while the carbon content of the control showed an increasing and then decreasing trend, the nitrogen content of both groups showed an increasing and then decreasing trend, and the C/N trend of the two groups showed a “stable-decreasing-increasing” trend. At the stage of flower bud physiological differentiation, the C/N content of terminal buds increased more than that of the control after KClO₃ treatment, while the N content of both groups did not change much. The C/N content of terminal buds increased from 22.82 to 23.41, while that of the control decreased from 22.82 to 22.63, showing a smooth trend in both groups. During the inflorescence formation stage of flower buds, the increase in total carbon content of terminal buds was smaller than that of the control, while the increase in total nitrogen content was larger than that of the control after potassium chlorate treatment; the C/N of terminal buds decreased from 23.41 to 17.65 after potassium chlorate treatment, while that of the control decreased from 22.63 to 20.29, and the decrease of terminal buds was larger after potassium chlorate treatment than that of the control. At the stage of inflorescence emergence, the whole carbon and nitrogen contents of terminal buds showed an increasing trend after potassium chlorate treatment, while the opposite was true for the control. The C/N of terminal buds increased from 17.65 to 21.44 after potassium chlorate treatment, while that of the control increased from 20.29 to 22.92, but the rate of increase of terminal buds was greater after potassium chlorate treatment than that of the control. This study reveals that a more than 21.47% increase in the C/N ratio of longan terminal buds is conducive to flowering. The application of potassium chlorate through soil leads to a continuous increase in total carbon content and an initial increase followed by a decrease in total nitrogen content in longan terminal buds, resulting in a significant rise in the C/N ratio, which is beneficial for the development of terminal buds towards floral formation.