The invention discloses the application of koumine to the preparation of medicines for treating chronic pains and belongs to the application of gelsemium alkaloid monomer. The analgesic experiments of the koumine on chronic pains of animals prove that the koumine has potent dose-dependent analgesic effect, the analgesic potency is superior to that of the classical antipyretic-analgesic and anti-inflammatory drugs which are aspirin and indomethacin, the therapeutic index of the koumine is much higher than that of the total alkaloids of gelsemium, the koumine possibly has no tolerance, no addiction and little side effect, which indicates that the koumine has potent and low-toxicity analgesic effect, the mechanism of action of the koumine is possibly different from those of clinically common opium analgesics or aspirin analgesics, the koumine and the pharmaceutically acceptable salt thereof have the application of preparing of the medicines for treating chronic pains and have no serious disadvantages of the clinically common analgesics and can be developed into a novel analgesic which has potent analgesic effect in treating chronic pains such as inflammatory pain, neuropathic pain, cancer pain and the like, has no tolerance and addiction and little side effect, thus the koumine has bright industrialization prospect.