The uses of guanidine hydrochloride as a drug for treating cancers/tumors. Guanidine hydrochloride can destroy the formation of hydrogen bonds by polar molecules on the surface of cancer cells and the macromolecules taken up, thereby demonstrating that guanidine hydrochloride can inhibit the uptake by cancer cells. The present invention provides new uses of guanidine hydrochloride in the treatment of subjects suffering from cancers/tumors and prevention of recurrence after cancer operation. The dose of guanidine hydrochloride administration is 10-35 mg/kg a day. The hydrochloride dose of the guanidine has a significant inhibitory effect on the uptake of DNA by cancer cells, which in turn can effectively inhibit the proliferation of cancer cells; besides, the inhibition rate of liver cancer tumors 9 days after the injection of guanidine hydrochloride drugs can reach 42%, and the inhibition rate of lung cancer tumors 7 days after the injection of guanidine hydrochloride drugs reach 56.8%.