Disclosed herein are polypeptides or their derivatives and their application. The polypeptides and their derivatives can treat or prevent cancer. The polypeptides of the invention have significant lethality to the cancer cells when used alone. When its clinical commonly used chemotherapy drugs such as cisplatin in combination, it can significantly increase the sensitivity of chemotherapeutic agents on cancer cells, to enhance its lethality of cancer cells, to reduce the dosage. The peptides can kill a variety of cancer cells, but without apparent toxicity enhancing effect on normal cells. The prepared peptides of the present invention can be chemically synthesized, high-purity, low molecular weight, specificity, non-immunogenic, safe and reliable.