To simplify the structure and reduce the high energy consumption of thermal insulation structures for polysilicon directional solidification equipment, an innovative cavity structure with a mirror and diffuse reflector was designed for thermal insulation. This method uses an internal thermal radiation cavity instead of a traditional insulation layer. Compared with the traditional insulation material, its volume was only 16.6 % of the crucible volume, and the energy consumption was reduced by 8.6 %. A mathematical model of the thermal radiation exchange in the cavity was established and solved analytically. Because of the design of mirror and reflector, the self-adjustment of the thermal radiation intensity and angle in the cavity was theoretically verified. It could automatically “trail” the temperature change on the sidewalls of the melt (crucible), and the dynamic thermal compensation was equivalent to an adiabatic construction. Three comparison experiments with a large ingot (0.9 m × 0.9 m × 0.35 m) were performed. When the cavity height to width ratio was greater than 3.33, the thermal compensation was poor and the quality of the ingot is lower than that of the others. The experiments showed that refractory brick and polished brass are a good combination for creating a cavity.