A radiation therapy medical apparatus is disclosed. The medical apparatus comprises: a base; a cylindrical gantry, peripherally and rotatably supported by the base; a radiation therapy assembly, comprising an arm and a radiation head, wherein one end of the arm is fixed to a first position on a first side of the gantry and the other end thereof is extended outwardly, and the radiation head is fixed to the other end of the arm; an imaging assembly, mounted to a second side of the gantry opposite to the first side, and configured to be a first balanced weight part for balancing the radiation therapy assembly; and a counterbalance, fixed to the second side of the gantry, and configured to cooperate with the imaging assembly to prevent the gantry from turnover under action of the radiation therapy assembly and configured to dynamically balance with the radiation therapy assembly with respect to a rotation axis of the gantry.