Integrating the design of insulation, heat storage, and solar heating can help to improve indoor thermal environment during winter in Western China. However, insulation at night is often overlooked for Trombe walls (TWs). The present study considers a panel curtain installed in the cavity to enhance the preservation of heat of TWs at night. The effects of the structural parameters related to insulation inside the cavity on the TWs were analyzed in Western China and a design approach for insulation inside the cavity was proposed. In addition, the indoor air temperature of a TW with a built-in panel curtain (CTW) mom was analyzed. The results indicate that the heat efficiency in the nighttime can be significantly improved by setting a panel curtain in the air cavity. Low emissivity film should be attached to the closed cavity side. The emissivity of the panel curtain's exterior surface plays an important role in the closed cavity and the effect is more significant with low emissivity. The optimum thickness depends on the climate, i.e., 0.02-0.06 m for a large solar radiant heat loss ratio (SHLR), 0.02-0.05 m for a medium SHLR, and 0.02-0.04 m for a low SHLR. The thickness of the circulated cavity should be larger than 0.07 m, and the effect varies within a small range when the thickness exceeds 0.1 m. A panel curtain can significantly improve the indoor air temperature in a CTW mom.