The invention provides an application of doxycycline in the preparation of antitumor drugs. The invention discloses that doxycycline has an antitumor activity, and is capable of inhibiting the tumor growth, tumor transfer and tumor invasion. Through the inv-vitro cellular level experiments and animal level experiments, people find that doxycycline can inhibit the growth and transfer of malignant tumors such as breast cancer, liver caner, melanoma, lung cancer, pancreas cancer, colon cancer, and the like. At the same time, the antitumor effect of doxycycline on a tumor-bearing mouse model is also very good. Moreover, doxycycline has multiple unique advantages, and can be accepted by patients more easily, and the drug effect on patients can be conveniently observed. The conventional tumor chemotherapy drug market will be changed by doxycycline. Doxycycline can be used regularly, and is a clinical drug capable of effectively inhibiting tumor transfer, invasion, and relapse.