The invention provides a slimming tablet and a preparation method thereof, and relates to a functional food. Main components of the slimming tablet are tartary buckwheat, konjak, L-carnitine and chitin. The slimming tablet is prepared by a certain process, and is used for absorbing intestinal fat, transferring and burning fat in a body, and adjunctively treating obesity, in cooperation with proper exercise. The slimming tablet is characterized in that tartary buckwheat can reduce blood fat; the konjak can clean intestines and stomach and reduce human body triglyceride level; chitin can reduce the content of triglyceride in blood and absorb the fat in the intestines; and the L-carnitine can transfer and burn the fat in the body. Functional experiments demonstrate that the function of insulin secretion of pancreas of the diabetes can be recovered completely and the level of fasting blood glucose, blood sugar two hours after meals, triglyceride, cholesterol, glycosylated hemoglobin can be recovered to normal by taking the slimming tablet in a proper amount according to requirements. The slimming tablet has the following advantages that 1. the slimming tablet contains pure natural nutrients; 2. the slimming tablet has no side effects, is safe and reliable and can be taken for a long time; 3. the slimming tablet has a significant effect; 4. the slimming tablet can replace the meals and is easy to take; 5. the slimming tablet is convenient for storing, preserving and carrying; and 6. the slimming tablet has a synergetic effect for different stages.