Disclosed is a beam limiter, comprising a rotary collimator. The rotary collimator comprises a truncated cone-shaped housing that has a central cavity and that has several rotation collimating holes, which have different apertures and are parallel to the generatrix, radially distributed on a side surface thereof; a drive device for driving the truncated cone-shaped housing to rotate; an angle detection device for detecting the angle of the truncated cone-shaped housing; and a control device connected to the drive device and the angle detection device respectively and used for receiving a signal sent by the angle detection device and controlling the starting or stopping of the drive device based on the received signal. In the usage of the collimator, the angle detection device and the control device together control the automatic rotation or stopping of the drive device, and the truncated cone-shaped housing automatically rotates or stops therewith to complete the change of a radiation field, thereby eliminating the repeated replacement of the collimator. Therefore, the radiation field can be easily, quickly and automatically changed by the collimator provided in the present application. Further disclosed is a radiotherapy apparatus comprising the beam limiter.L'invention concerne un limiteur de faisceau, comprenant un collimateur rotatif. Le collimateur rotatif comprend un boîtier en forme de cône tronqué qui a une cavité centrale et qui a plusieurs trous de collimation de rotation, qui ont des ouvertures différentes et qui sont parallèles à la génératrice, répartis radialement sur une surface latérale de celui-ci; un dispositif d'entraînement pour entraîner le boîtier en forme de cône tronqué en rotation; un dispositif de détection d'angle pour détecter l'angle du boîtier en forme de cône tronqué; et un dispositif de commande connecté au dispositif d'entraînement et au dispositif de détection d'angle respectivement et utilisé pour recevoir un signal envoyé par le d