To analyze the effects of radial temperature gradients on the surface figures of lightweight reflective mirrors, a 2-m-aperture lightweight mirror of a space camera was taken as the research object and a finite element model of the mirror was established. The effects of different radial temperature gradients and lightweight structures on the surface figure Root Mean Square (RMS) values of reflective mirrors was analyzed, and the accuracies of a simulation method and conclusions were verified through an experiment. In addition, the coupling effects of different lightweight structures and radial temperature gradients on the mirror surface figure were studied. Results show that the surface figure RMS values derived from the radial temperature gradients of different distribution modes can differ by a maximum of 294 times, and the surface shape error caused by the radial gradient is difficult to reduce when optimizing the mirror support scheme. The mirror surface figures of different lightweight structures are different for the radial temperature gradient sensitivities of different distribution modes. The lightweight methods of trefoil and symmetric chamfering are thermally stable to the mirror in a specific radial temperature field distribution mode. Finally, sensitivity has an adverse effect.