The invention belongs to the technical field of food processing and specifically relates to cassia seed and citrus aurantium flower oral liquid and a preparation method thereof. The cassia seed and citrus aurantium flower oral liquid is mainly prepared from the main raw materials in parts by weight, including 30-50 parts of cassia seed, 20-40 parts of citrus aurantium flower, 10-20 parts of rose, 3-5 parts of stevia rebaudiana, 3-5 parts of fruit of Chinese wolfberry, 3-5 parts of liquorice, 0.5-2 parts of passion fruit, 0.5-2 parts of aloe and 0.5-2 parts of loofah stems, through the processes of sprouting and roasting the cassia seed, CO2 subcritical extraction, juicing, mixing, filtering, canning, sterilizing, and so on. The cassia seed and citrus aurantium flower oral liquid is formed by scientific mixing of pure traditional Chinese medicines, the prescription is rational, the technology is advanced, the product has the characteristics of having remarkable effects of reducing fat and losing weight, having no toxic effect, sustaining weight loss, being suitable for long-term use, and so on; moreover, the product has healthcare functions of relieving liver for smoothing Qi, beautifying and moistening skin, refreshing mind, soothing nerves, clearing away heat and toxic materials, reinforcing qi and nourishing blood, and also has taste and flavor of coffee; taste and flavor of the product both are excellent, raw material resources are abundant and easily available, and thus the cassia seed and citrus aurantium flower oral liquid has great market application and generalization value.