FLASH radiotherapy is a hotspot in the domain of tumor radiotherapy in recent years, which delivers at ultra-high dose rate (usually > 100 Gy/s) in an ultra-short time (1−50 ms) to the target volume. The FLASH effect will be generated after the organism is treated with FLASH radiotherapy, which makes the tumor more easy to be killed and the normal tissue is protected after radiotherapy. Because of the differences in sensitivity to FLASH radiotherapy between tumor tissues and normal tissues, FLASH radiotherapy has a subversive advantage in the treatment of tumors. In this paper, several studies since 1959 on the effects of ultra-high dose rate rays and FLASH radiation on cells and organisms are summarized. As the predecessor of FLASH radiotherapy, ultra-high dose rate radiotherapy has laid a very important foundation for the development of FLASH radiotherapy.