The invention discloses a protection device for applying intracavity after-loading treatment to gynecological malignant tumors. The protection device comprises a fixing plate, a threaded sleeve, an avoiding hole, a sliding column, a fixing frame A, a water injection blocking bag A, a fixing block, a T-shaped column, a threaded rod, a connecting frame, a fixing frame B, a water injection blocking bag B, a Y-shaped water pipe and a water pump. According to the protection device for applying intracavity after-loading treatment to gynecological malignant tumors, firstly, through the ingenious design of an expansion mechanism, the vagina area of a gynecological patient can be opened, and inspection and operation in the vagina are facilitated; the fixing effect is achieved in cooperation with the effect of the fixing frame; secondly, the bladder and rectum area of the patient can be well blocked and protected and the irradiation dose endangering organs can be reduced in cooperation with the effect of the liquid injection type water bag blocking structure, thereby ensuring after-loading treatment safety; and in addition, the problems that the accuracy of the irradiation dose of a target region is reduced, the repeatability is poor and the like due to the fact that the existing clinical gauze packing is prone to position inconsistency each time are solved, and finally popularization and application of gynecologic tumor treatment are facilitated.