Immunotherapy is a promising field in cancer treatments, since it combats cancer by strengthening the host's natural defenses. Based on the targets of cancer immunotherapy, there are two strategies to achieve cancer elimination. The first strategy directly modifies immune effector cells to boost their tumor-killing activity. The other way aims to prevent cancer immune evasion by targeting cancer cells and/or the surrounding tumor microenvironment. The most successful immunotherapeutics such as CAR T cells and immune checkpoint inhibitors belong to these two types, respectively. Compared to traditional cancer treatments like chemotherapy, radiotherapy and surgery, the newly raised immunotherapy has advanced specificity, prolonged responses and more efficient outcomes. This review discusses the progression of cancer and the mechanisms of failed natural defense against cancer, also highlights the principles, current status, barriers for further applications and future perspectives of cancer immunotherapy.