A use of a polymer as a synergist for a chemotherapeutic drug, wherein the polymer is a polymer having a 1,2-dicarboxylic acid monoamide structure, and after the polymer, a chemotherapeutic drug and a cancer cell are co-incubated in a slightly acidic environment which simulates a tumor tissue, a survival rate of the cancer cell is lower than that of a cancer cell incubated under the same conditions without the polymer. The polymer and the chemotherapeutic drug are injected together into a tumor-bearing animal body, the tumor-growth inhibiting effect thereof is better than that with injection of the same dosage of the chemotherapeutic drug alone.
