The invention provides a radiation therapy device which comprises a base, an O-shaped arm, a therapy head, a therapy couch and a chassis. The chassis is arranged below the base. The base can rotate around the axis of the chassis. An arc-shaped track B is arranged on the inner side of the O-shaped arm, wherein the axis of the arc-shaped track B is located on the plane of the O-shaped arm. The therapy head comprises a moving device. One end of the moving device is arranged in the arc-shaped track B, and can rotate along the arc-shaped track B. The other end of the moving device is fixedly connected with a ray source, and can drive the ray source to carry out telescopic movement along the radial direction of the O-shaped arm to adjust the distance between the ray source and the center of the O-shaped arm. According to the radiation therapy device provided by the invention, three rotation freedom degrees are used to adjust the incidence angle of a ray to realize non-coplanar therapy; the radiation ray source is designed to be scalable, which realizes distance-adjustable radiotherapy to lesions; and the radiation therapy device has the advantages of large radiation ray source moving range, high flexibility, small ray attenuation and the like.