After radiotherapy for rectal cancer, the tumor may shrink in about 2 weeks, but not all patients’ tumors shrink at the same time. If the tumor cells are more sensitive to radiotherapy and the treatment effect is better after rectal cancer has undergone radiotherapy, the tumor cells may be necrotic and absorbed in large quantities in about 2 weeks, and the lesions begin to shrink. It should be noted that there are certain individual differences in rectal cancer radiotherapy. If the tumor is more serious rectal cancer patients and not very sensitive to radiotherapy, the treatment effect is relatively poor, the tumor shrinkage will be slower, or even will not be shrunken, it is necessary to combine with imaging examination, with systemic chemotherapy, local intervention therapy, etc., to control the condition in time. Therefore, whether the tumor will shrink after radiotherapy for rectal cancer and the time of shrinkage are mainly related to the tumor severity, radiotherapy sensitivity and other factors. In the process of specific radiotherapy, it is recommended to listen to the radiotherapy advice of professional doctors and make reasonable radiotherapy and comprehensive treatment plan.