The invention discloses a medicine for treating tumors and an application of the medicine. The medicine is N-methylcytisine or a mixture of N-methylcytisine and 5-fluorouracil. With the application of the medicine, the proliferation of human hepatoma carcinoma cells SMMC-7721 and murine lung cancer cells LLC can be obviously inhibited. The mixture of the N-methylcytisine and the 5-fluorouracil provided by the invention, in comparison with a classic anti-tumor medicine which is independently used, namely the 5-fluorouracil, shows a better anti-tumor effect; therefore, the medicine has relatively good application prospect and clinical reference value.