The heat storage performance of internal envelope plays an important role in improving indoor thermal comfort for a passive solar room. Therefore, how to design thermal properties of building internal envelope should be studied. The previous researches are all case studies, which means the conclusions can not be used generally, and the intrinsic performance can not be known. The paper aims to analyze the intrinsic performance of internal envelope controlling indoor temperature and optimize the specific heat. The results show that the optimal specific heat of internal envelope is nearly δ function which approaches to the effective heat capacity form of phase change material (PCM). The paper is helpful to understand the temperature control performance of internal envelope deeply and obtain the optimal specific heat directly.