The invention provides a small reflector ultrahigh power thermal stability supporting structure, and belongs to the technical field of the aerospace technology. The supporting structure solves the problems that in the prior art, a reflector is low in intrinsic frequency and poor in external disturbance resistance, a reflection mirror surface is large in shape change, the requirement for the assembly technique level is high, interchangeability is poor and mass production is difficult to achieve. The supporting structure comprises a mirror chamber and a flexible joint, the flexible joint and the reflector are in glue joint and fixed concentrically, the flexible joint is fixed in the mirror chamber and comprises a cylindrical barrel and multiple flexible single arms which are uniformly distributed on the outer side of the cylindrical barrel in the circumference direction and each flexible single arm is a double-layer flexible hinge. Under the action of temperature loads, in the process that thermal deformation stress deformation energy of the lens barrel is transmitted to the reflector, the thermal deformation stress deformation energy is absorbed when passing a flexible unload groove of the flexible joint, uniform deformation of the reflector is guaranteed, at the moment, surface precision slightly changes, the parts are fixedly connected, the requirement for the assembly technique is not high, and the supporting structure is good in interchangeability and high in external dynamic interference resistance.