There are 3 lines of defense in the prevention of malignant tumors, called 3 levels of preventive measures. The first line of defense is etiological prevention, also known as primary prevention. Eliminate the risk factors and causes of disease, improve cancer prevention ability, and prevent the disease before it occurs. Known risk factors such as smoking, alcoholism, unnecessary radiation exposure and occupational exposure should be controlled and eliminated by taking appropriate measures. For example, not smoking in public places, prohibiting teenagers from smoking, stipulating that cigarette tar in paper cigarettes should be reduced to less than 15mg per cigarette, etc. In addition, it is also necessary to improve the body’s ability to fight against cancer, and to carry out preventive vaccination or chemoprophylaxis. For example, newborns in areas with high incidence of liver cancer should be vaccinated against hepatitis B. Improving diet and nutrition is also a cause of the disease. Improving diet and nutrition is also one of the main elements of etiologic prevention. For example, high-fat diet may be related to breast cancer, colon cancer and prostate cancer. Therefore, the calories from fat in people’s diet should not exceed 30% of the total calories. In order to prevent the occurrence of esophageal cancer and stomach cancer, salt, salt smoked and nitrate food should be reduced. More fruits, vegetables, foods rich in vitamins A and C and fiber should be promoted. When carrying out primary preventive measures, it is common to encounter cases where the etiology is not clear but there is evidence of risk factors. Preventive measures can also be carried out first to observe the effect of prevention, while laboratory studies can be conducted to find out the cause of the disease. The second line of defense is early detection, diagnosis and treatment, also known as level 2 prevention. This is a preventive measure, i.e., when the tumor just starts to occur, early screening and treatment can be carried out to get twice the result with half the effort. 2-level preventive measures actually include two aspects: one is early detection, i.e., medical workers go deep into the population and find out the early stage cancer patients by means of effective screening; the other is that for the suspected patients found by screening, doctors should give the diagnosis and treatment as timely and accurately as possible. treatment. Cancers that are more effective in level 2 prevention are cervical cancer and breast cancer. Other tumors can be screened if they pose a greater threat to people’s health, have a clearer medical history, have a basic early diagnosis, have a better early treatment effect, do not cause damage to the examinee, and do not cost much. The third line of defense is rehabilitation prevention, also known as level 3 preventive measures. Rehabilitation is carried out for tumor patients after treatment by various methods to reduce complications, prevent disability, and improve survival rate and quality of life. Pain relief and end-of-life care should be implemented for advanced patients. All in all, cancer patients should be cared for from physiological and psychological aspects. Now, clubs, anti-cancer associations, schools and other organizations have been set up in different places, inviting medical personnel to carry out regular follow-ups and re-examinations for patients after treatment, instructing them on diet, hygiene, labor, life, discouraging smoking, alcoholism, correcting bad habits of life and diet, consulting with them on all aspects of problems, and giving them necessary treatments in time, so as to improve the quality of survival and prolong the time of their survival. It is evident from the above that there are many preventive measures against cancer, involving a wide range of aspects, and the real work must be done in accordance with the situation of each individual, and the priorities must be clearly differentiated. Over the years, cancer prevention, both theoretically and practically, has accumulated a great deal of experience, and by taking effective measures, malignant tumors will certainly be controlled in the near future.