A novel rigid support structure is proposed in this paper to solve the contradiction between thermal stability and structural stiffness in small- and medium-aperture space mirrors assembled using traditional flexible supports. Additionally, a high-precision secondary mirror assembly with a clear aperture of ?214 mm is developed for a high-resolution space camera. The combination of a mirror body, cone, support cylinder, and rigid base plate is adopted to realize heat dissipation by extending and optimizing the transmission path of the thermal stress within the assembly. The secondary mirror assembly with a rigid support structure weighs 2.6 kg, and the surface accuracy change has a root-mean-square (RMS) value of 2.573 nm in the simulation under the condition of a 4 °C uniform temperature rise. The inclination and displacement of the mirror body subjected to the gravity test are 2.028" and 0.566 μm, respectively, revealing the outstanding advantages of the proposed scheme over traditional flexible support systems. The measured surface accuracy RMS value of the secondary mirror is 0.0181λ (λ=632.8 nm), and the changes in the surface accuracy at 16 and 24 °C do not exceed 0.0025λ. The fundamental frequency of the assembly reaches 502.1 Hz, and the surface accuracy of the secondary mirror remains relatively unchanged after rapid heat cycles and large-scale vibrations. In the assembling tolerance test, the secondary mirror is only slightly deformed under 0.02 mm unevenness. The proposed rigid support structure can significantly improve the working performance of small- and medium-aperture mirrors and has broad application prospects in the optomechanical structural design of remote sensors.